Chem 140b Notes

8/4/2019 Chem 140b Notes

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CHAPTER 10 Using Nuclear Magnetic Resonance Spectroscopy to Deduce Structure

10-1 Physical and Chemical Tests

Some common physical tests are melting point and boiling point. Chemical tests such as elemental analysis

can help define the structure of an unknown compound. However, there are literally millions and

millons of organic compounds and different structures can have the same physical and chemical

properties.

10-2 Defining Spectroscopy

Spectroscopy involves the analysis of the absorption of radiation.Spectroscopy detects the absorbtion ofenergy by molecules. Absorption of infrared energy results in stretching and bending of bonds. Visible

and ultraviolet light are absorbed and result in the excitation of electrons from lower to higher lying

orbitals. In the presence of a magnetic field, certain nuclei absorb electromagnetic radiation resulting in

a re-alignment with the applied magnetic field.

E = hv

Goes from ground state to excited state from absorbption of E.

v = c/

v=frequency

c=speed of light (299 792 458 m/s)

= wavelength

Decreasing

Increasing v

Increasing E

Example : The to initiate radical chlorinization uses the energy to break Cl-Cl (58 kcal/mol). It isfound by =c/(58 kcal/mol) = 493nm

10-3 Proton Nuclear Magnetic Resonance

NMR is possible with certain nuclei such as 1H and 13C exposed to a strong magnetic field. They align () with or ()

against it. to transitions lead to resonance and a spectrum with characteristic absorption. The higher the

external field strength, the higher the resonance frequency.

Ex: Frequency of 7.05 T causes hydrogen to absorb 300MHz, a magnetic field of 14.1 T causes it to absorb at 600

MHz.

A proton spectrum provides information as to:

1. electron density (further left is more electron defficient)2. number of hydrogens (from area of peaks)3. number of neighboring hydrogens (from peak multiplicity)

10-4 Using NMR Spectra to Analyze Molecular Structure: The Proton Chemical Shift

High Resolution NMR allows for differentiation of hydrogen and carbon nuclei in different chemical

environments.

Chemical shift () in ppm is the position in the spectrum of H and C nuclei in relation to the internal standard,

tetramethylsilane(Si(CH3)4).

The energy required to reorient a hydrogen nucleus in a magnetic field depends on the electron density

surrounding the hydrogen.

The number of resonance signals in spectrum - number of sets of hydrogen atoms in different chemical

environments.


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Chemical shift Hydrogen Type

0-1.5 ppm Alkane-type

1.5-3.0 ppm on carbon next to carbon-containing functional groups

3.0-4.5 ppm on carbon attached to electronegative atoms

4.5-6.0 ppm Alkene-type

6.0-9.5 ppm Benzene-type

9.5-10.0 ppm Of aldehyde group

10-5 Tests for Chemical Equivalence

Chemically equivalent hydrogens will appear in an NMR spectrum at the same place. However, chemically different

hydrogens may or may not appear as distinct signals.

10-6 Integration

The intensities of signals for hydrogen atoms in an NMR spectrum are directly proportional to the number of hydrogens

contribution to each signal.

10-7 Spin-Spin Splitting: The Effect of Nonequivalent Neighboring Hydrogens

A hydrogen atom with a neighboring hydrogen is exposed to two magnetic fields as the neighbor either

adds to or substracts from the applied magnetic field. This results in two unique signals. Two neighbors

created three unique magnetic fields, and three neighbors result in four unique fields. This multiplicity is

very useful in deducing the structure of a compound from its NMR spectrum.

10-8 Spin-Spin Splitting: Some Complications

Non-first order effects can complicate the analysis of NMR spectra.

The splitting patterns found in various spectra are easily recognized, provided the chemical shifts of the

different sets of hydrogen that generate the signals differ by two or more ppm. The patterns are symmetrically

distributed on both sides of the proton chemical shift, and the central lines are always stronger than the outer

lines. The most commonly observed patterns have been given descriptive names, such as doublet (two equal


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intensity signals), triplet (three signals with an intensity ratio of 1:2:1) and quartet (a set of four signals with

intensities of 1:3:3:1). Four such patterns are displayed in the following illustration. The line separation is always

constant within a given multiplet, and is called the coupling constant (J). The magnitude of J, usually given in

units of Hz, is magnetic field independent.

10-9 Carbon-13 Nuclear Magnetic Resonance

Carbon-13 represents only about 1% of the carbon in a sample, so instrumentation must be more sensitive than used for

proton spectroscopy. On the other hand, the probability of carbon atoms coupling with each other is small as it is

unlikely that a structure will have two carbon-13 atoms next to each other.

The range covered by C13 is much wider than is that for protons and, as a result, very rarely do unique carbon atoms

overlap. Thus, C13 spectra can be used to determine the number of unique carbon atoms.

C13 also couples to protons and from this, it can be determined the number of hydrogen atoms attached to each

carbon.


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CHAPTER 11 Alkenes; Infrared Spectroscopy and Mass Spectrometry

11-1 Naming of Alkenes- Alkenes are named so that both sp2 hybrdized carbon atoms are included in the longest chain. The chain is

number from the end that gives the first of these atoms the lowest number.

o 1-Butene has the double bond between carbon 1 and 2 (terminal alkene). 2-Butene has it between 2and 3 (internal alkene).

- When one or more substituents are present on each carbon of the alkene, stereoisomers are possible.- Two groups, one on each carbon, are on the same side, they are said to be cis.

o Conversely, when they are on opposite sides, they are called trans.- A more general system sets priorities for both substituents on each carbon based upon the Cahn, Ingold, Prelo

system. When the two higher priority substituents are on the same side, the alkene is called Z. Otherwise, it is

called E.

11-2 Structure and Bonding in Ethene: The Pi Bond

The two bonds present in an alkene are different: one is formed from the overlap of sp2 orbitals and is

call a sigma bond. The other results from overlap of to p orbitals and is called a pi bond.

Whereas there is relatively free rotation about sigma bonds, the pi bond locks the substitutents

on the two carbons into one plane that also contains the two carbons of the double bond.

Thhe pi bond is relatively weak

11-3 Physical Properties of Alkenes

The present of the double bond makes only minor changes in the physical properties of alkenes relative

to the alkane with the same connectivity of carbon atoms.

11-4 Nuclear Magnetic Resonance of Alkenes

Protons bound to sp2 hybridized carbon atoms of alkenes in the region for 5-7.

- Coupling between hydrogens that are trans is in the range of 11-18 whereas the range for cis is 6-14.o The presence of more electronegative substituents reduces coupling to the low end of the range.

- For hydrogens on the same carbon (geminal hydrogens) the coupling is small but real and in the range of 0-3.11-5 Infrared Spectroscopy

- Most functional groups have characteristic absorptions in the infrared region of the spectrum. Those that resultin bond stretching are the most characteristic.

- Hydrogenation, exothermic, heat released is per double bond.o C=C + H-H CH-CH Ho = -30kcal/mol(-125 kcal/mol)

- Stability : 1-butene < cis-2-butene < trans-2-buteneo CH2=CH2 < RCH=CH2 < cis HRC=CRH < trans HRC=CRH < CR2=CRH < CR2=CR2(most stability)o Increasing alkene stabilityo Decreasing heat of hydrogenation

- The most useful absorptions are for: carbonyl groups; nitriles; amines; and alcohols. Different types of carbonylgroups give rise to different frequencies.

11-6 Measuring the Molecular Mass of Organic Compounds: Mass Spectrometry

General Elimination : CA-CB C=C + AB- High resolution mass spectrometry can be used to determine the molecular formula of organic compounds .- Alkenes generally made by E2 reactions, which favor cis over trans.- Some E2 processes are stereospecific.

11-7 Fragmentation Patterns of Organic Molecules

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11-8 Degree of Unsaturation: Another Aid to Identifying Molecular Structure

Compounds containing only C, H, and O with no double or triple bonds and no rings will have a formula where the

number of carbon and hydrogen atoms will have the relationship: CNH2N+2. For each ring or double bond present, the


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number of hydrogen atoms decreases by 2. Thus, inspection of the molecular formula reveals the total number of rings

and double bonds present (triple bonds count as two double bonds).

11-9 Catalytic Hydrogenation of Alenes: Relative Stability of Double Bonds

Addition of hydrogen to an alkene can be achieve in the presence of a catalyst (Pt, Pd). Using average bond energies, the

reactions is exothermic by 31 kcal/mole.

Experimental heats of hydrogenation for the isomeric butenes reveals that trans-2-butene is the most stable, the cis

isomer is less stable by 1.0 kcal/mole and 1-butene is the least stable, being 2.7 kcal/mole less stable than is trans-2-

butene.11-10 Preparation of Alkenes from Haloalkanes and Alkyl Sulfonates: Bimolecular Elimination Revisited

E2 elimination reactions generally lead to mixtures as isomers (including cis/trans) are usually possible for the product

alkene. While the more substituted alkene is the more stable, there are more hydrogens that can be removed to form

the less substituted isomer(s) and therefore energetics works in the opposite direction as statistics.

11-11 Preparation of Alkenes by Dehydration of Alcohols

- Elements of Unsaturation- The presence of a pi bond or a ring in a compound decreases the number of H atoms in its molecular formula.

Each ring or pi bond decreases 2 H atoms, corresponding to a single element of unsaturation. Remember, a

fully saturated compound has molecular formula: CnH2n+2

- Thus, C4H8 has one element of unsaturation, which may indicate the presence of either one double bond orone ring. See the possibilities below:

Formula Representative Structures Degree of

Unsaturation

C6H14 Hexane 0

C6H12 Hexane with one (double) bond, one ring 1

C6H10 Hexane with 2 bonds, one ringwith one bond, 2 rings(pen-pro) 2

C6H8 Hexane with 3 bonds, one ring with 2 bonds, 2 rings with one bond 3


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2. The reaction adds the H and the OH groups cis to each other.12-9 Diazomethane, Carbenes, and Cyclopropane Synthesis

Carbenes are highly reactive, unstable carbon species with only two Substituents and only six-valence electrons.

They react with alkenes to form compounds with three-membered rings (cyclopropanes).

12-10 Oxacyclopropane (Epoxide) Synthesis: Epoxidation by Peroxycarboxylic Acids

Epoxides are formed upon treatment of alkenes with carboxylic acid peracids. The reaction is concerted with all

bond changes taking place in a single step.

12-11 Vicinal Syn Dihydroxylation with Osmium TetroxideReaction of OsO4 with alkenes results in the formation of osmate esters by a concerted reaction in which two of

the oxygen atoms of OsO4 add simultaneously to the two carbons of the alkene. The osmate ester is cleaved to

a diol with oxidation of the Os to the +8 state by a hydroperoxide, ROOH. The addition of the two oxygens is syn.

12-12 Oxidative Cleavage: Ozonolysis

Treatment of alkenes with O3 followed by reduction results in cleave of both the sigma and pi bond with each of

the original sp2 carbons of the alkene being carbonyl group carbons in the product(s). Cyclic alkenes result in a

single dicarbonyl group product whereas acyclic alkenes produce two molecules of carbonyl product. These can

either be the same or different depending on the starting alkene. With symmetrical alkenes, a single product is

observed.

http://www.organic-chemistry.org/namedreactions/ozonolysis-criegee-mechanism.shtm

12-13 Radical Additions: Anti-Markovnikov Product FormationIn the presence of radicals, HBr adds to alkenes with Anti-Markovnikov regiochemistry. The reactions begins

with the formation of RO radicals that abstract a hydrogen atom from HBr, production ROH and bromine

radical. The bromine radical adds to the less substituted carbon of the alkene, thus generating the more

substituted and therefore for stable carbon radical. This radical abstracts a hydrogen from HBr, generating the

organic product and another bromine radical that continues the process in a chain reaction.

12-14 Dimerization, Oligomerization, and Polymerization of Alkenes

The addition of a radical (e.g. a bromine radical) to an alkene forms a carbon radical while the pi bond is lost.

This radical can react with another molecule of alkene with the formation of a new CC bond and a new carbon

radical. This process results in the formation of a CC sigma bond at the expense of the pi bond and is thus

roughly 20 kcal/mole exothermic.

12-15 Synthesis of Polymers

12-16 Ethene: An Important Industrial Feedstock

12-17 Alkenes in Nature: Insect Pheromones
http://www.organic-chemistry.org/namedreactions/ozonolysis-criegee-mechanism.shtmhttp://www.organic-chemistry.org/namedreactions/ozonolysis-criegee-mechanism.shtmhttp://www.organic-chemistry.org/namedreactions/ozonolysis-criegee-mechanism.shtm


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CHAPTER 13 Alkynes: The Carbon-Carbon Triple Bond

13-1 Naming the Alkynes

1. Endingene replaced withyne2. Number indicates position of the triple bond (2 means bond between 2nd and 3rd carbon)3. Substituents with triple bond are called alkynyl groups. For alcohols it is called alkynols

a. HCCCH2OHis called 2-Propyn-1-ol4. Hydrocarbons with both double and triple bonds are alkenyne.

13-2 Properties and Bonding in the AlkynesThe triple bond of alkynes is made up of a sigma bond (from spsp overlap) and two pi bonds (from pp orbital

overlap). The sigma bond is stronger (83 kcal/mole) and the two pi bonds in total contribute 117 kcal/mole.

Removing one on the pi bonds requires 54 kcal/mole. Terminal alkynes are relatively acidic, with pKa values of

about 25.

o Hybridization affects Acidity H3C-CH3 < H2C=CH2< HCCH sp3 (50) sp2 (44) sp (25)

o13-3 Spectroscopy of the Alkynes

Alkynes have a weak absorption at 2100 but only unsymetrical alkynes absorb. The proton of terminal alkynes

is shielded by the triple bond and occurs at approximately 2 ppm. Hydrogens on carbons bound to the sp

carbons of an alkyne are deshield and occur at approximately 2.5.

13-4 Preparation of Alkynes by Double Elimination

Addition of Br2 to an alkyne followed by treatment with a strong base such as NaNH2 results in the formation of

an alkyne where the triple bond is between the two carbon atoms that were sp2 in the alkene.

13-5 Preparation of Alkynes from Alkynyl Anions

The anion resulting from deprotonation of a terminal alkyne acts as a nucleophile reacting with alkyl halides

(and sulfonate esters) in an SN2 reaction. Yields are good with methyl and primary alkyl halides.

13-6 Reduction of Alkynes: The Relative Reactivity of the Two Pi Bonds


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Reduction with H2/Pd of an alkyne to an alkene is faster than that of an alkene to an alkane. However, both

reactions are fast and for practical control of the reaction, Lindlar catalyst is used. Lindlar catalyst is Pd on CaCO3

that has been poisoned with Pb(OAc)2 and quinoline, an organic base.

13-7 Electrophilic Addition Reactions of Alkynes

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13-8 Anti-Markovnikov Additions to Triple Bonds

Alkynes undergo addition of water in the presence of strong acids (practically, HgSO4, a Lewis acid, is added to

accelerate the reaction). The resulting product is a ketone. The reaction is generally limited to symmetrical

alkynes as otherwise, two different ketones will be formed in roughly equal amounts.


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CHAPTER 14 Delocalized Pi Systems: Investigation by Ultraviolet and Visible Spectroscopy

14-1 Overlap of Three Adjacent p Orbitals: Electron Delocalization in the 2-Propenyl (Allyl) System

The CH bond on carbon adjacent to a pi bond is unusually weak (87 versus 101 kcal/mole). This decrease on bond

strength is the result of an increase in stability of the resulting allylic radical as compared to an ordinary radical. Allylic

radicals are stabalized by resonance where the unpaired electron is distributed over two carbon atoms. In molecular

orbital terms, the combination of three p orbitals results in the fromation of three molecular orbitals: bonding;

nonbonding; and antibonding. The bonding orbital has no nodes end-to-end, the nonbonding has one, and the

antibonding has two. Two of the three electrons of the allylic radical are in the bonding orbital and the odd electron is inthe nonbonding orbital.

14-2 Radical Allylic Halogenation

Because an allylic CH bond is unusually weak, free radical abstraction of such a hydrogen is faster than a non-allylic

hydrogen. Thus, free radical halogenation is selective for the allylic position. However, the resulting radical has two

carbons that can abstract a halogen atom and only when the radical is symmetrical is a single product obtained.

14-3 Nucleophilic Substitution of Allylic Halides: SN1 and SN2

SN1 reaction of an allylic halide is facilitated because the intermediate cation is resonance stabilized. Because there are

two carbons that bear positive charge in the intermediate cation, only symmetrical cations produce a single product. The

SN2 transition state is also stabilized by overlap of the p orbital on the carbon undergoing substitution with the pi

system of the adjacent double bond.

14-4 Allylic Organometallic Reagents: Useful Three-Carbon Nucleophiles

Allylic CH bonds can be deprotonate by strong base, for example BuLi. The resulting anion is resonance stabilized, with

negative charge on two carbons. This anion is nucleophilic and adds to, for example, ketones and aldehydes. The result

products are bifunctional, having an alcohol and an alkene.

14-5 Two Neighboring Double Bonds: Conjugated Dienes

Two adjacent double bonds are said to be conjugated. The four electrons of the pi system are not localized betweenpairs of carbon atoms. Rather, two are spread from end to end in an orbital that results from the combination of all four

of the p orbitals in phase. The remaining two electrons are also spread over the entire chain of four carbon atoms but

have a node in the middle between carbons 2 and 3. This delocalization results in additional stabilization for conjugated

dienes.

14-6 Electrophilic Attack on Conjugated Dienes: Kinetic and Thermodynamic Control

Addition of a proton to a conjugated diene lead preferentially to the conjugated cation. Bromide ion addition to this

cation is kintically favored at the more substituted carbon atom whereas equilibration of the allylic bromide products


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favors the product with the more substituted double bond. The reaction is said to be under kinetic control in the first

case and thermodynamic control in the second.

14-7 Delocalization among More tha Two Pi Bonds: Extended Conjugation and Benzene

Extended conjugation with three double bonds leads to six molecular orbitals, 3 bonding and 3 antibonding. As the

number of double bonds increases, the lowest bonding orbital becomes increasingly stable. In addition, as the number

of double bonds increases, the energy difference or gap between the HOMO and LUMO decreases. Benzene represents

a special case of conjugation where formally there are three double bonds within a six-membered ring. In realily, the

electrons are distrubuted evenly around the ring, leading to a very significant increase in stability for benzene and otheraromatic compounds.

14-8 A Special Transformation of Conjugated Dienes: Diels-Alder Cycloaddition

The Diels-Alder reaction combines a conjugated diene with an alkene (called a dienophile) in a reaction that forms a six-

membered ring by a concerted pathway. The presence of an electron-withdrawing group (EWG) such as a carbonyl

containing functionality on the dienophile double bond greatly increases the rate of reaction.

14-9 Electrocyclic Reactions

Electrocyclic reactions are similar to the Diels-Alder reaction in that they have a cyclic transition state and a concerted

mechanism.

14-10 Polymerization of Conjugated Dienes: Rubber

14-11 Electronic Spectra: Ultraviolet and Visible Spectroscopy


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CHAPTER 15 Benzene and Aromaticity: Electrophilic Aromatic Substitution

15-1 Naming the Benzenes

Proper, IUPAC names for substituted benzene compounds use numbers to indicate relative positions of

substituents. When only two substituents are present the terms ortho, meta, and para are commonly used instead.

- Non-Systematic Names:

- Generally, mono-substituted benzenes are named similar to hydrocarbons withbenzene as the parent name.

15-2 Structure and Resonance Energy of Benzene: A First Look at Aromaticity

Were benzene simply cyclohexatriene, it would be expected that reduction would release three times the

energy released upon reduction of cyclohexene, or 3 x28.6 =85.8. However, the observed value is 49.3 and

thus benzene is some 36 kcal/mole more stable that predicted for cyclohexatriene.

15-3 Pi Molecular Orbitals of Benzene

The molecular orbitals of the pi system of benzene are derived by combination of the six p-orbitals on the six

carbon atoms. All six molecular orbitals have a nodal plane that includes the carbon atoms as well as the

hydrogen atoms. The lowest lying orbital has no other nodes whereas the next two, equal energy orbitals have

one additional node. The nodes of these two orbitals are at right angle to each other. These three lowest lying

orbitals constitute the bonding molecular orbitals and hold the six-pi electrons of benzene. The relative energies

of orbitals of cyclic systems with sp2 atoms can be simply predicted by drawing the polygon of the structure

with one atom at the bottom. The vertices of the polygon are then located so as to represent the energy levels

of the orbitals.


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15-4 Spectral Characteristics of the Benzene Ring

The infrared spectrum of benzene compounds with two or more substituents show characteristic patterns that can beused to determine where the substituents are on the ring. However, analysis by proton and carbon nmr is now generally

used for this purpose. In the nmr experiment, the cyclic electron of the pi system of benzene create a magnetic field that

is shielding above and below the ring and deshielding outside the ring. As a result, protons on the ring resonate

downfield from those of simple alkenes.

15-5 Polycicylic Aromatic Hydrocarbons

Multiple aromatic rings can be present in a molecule. Examples are biphenyl, where two benzene rings are linked by a

CC sigma bond, and naphthalene, where two benzene rings are fused to each other and share two carbons between

them. Naphthalene does not have twice the stabilization energy of benzene even thought it formally has two such rings.

From heats of hydrogenation, naphthalene is 61 kcal/mole more stable than if the pi electrons were contained in simple

double bonds.

15-6 Other Cyclic Polyenes: Huckel's Rule

Molecules with rings of all sp2-hybridized atoms are aromatic if the have 2, 6, 10 etc electrons in the pi system. A simple

rule, called Huckel's rule, states that the required number of electrons for a compound to be aromatic is 4n + 2, where n

is an integer.

15-7 Huckel's Rule and Charged Molecules

Cyclic collections of sp2 atoms have a single lowest lying orbital. Orbitals then are in pairs of identical energy. For such a

system to be aromatic, there must be sufficient electrons to fill a level. Thus, 2, or 6, or 10 etc. Huckel's rule summarizes

this with the formula: 4n + 2, where n is an integer. The atoms involved need not be carbon, as in pyrrole, and the

molecule can be charged, as in the anion derived by deprotonation of cyclopentadiene.

15-8 Synthesis of Benzene Deriviatives: Electrophilic Aromatic Substitution

Powerful electrophiles react with aromatic compounds where 2 of the pi electrons are used to form a bond to the

electrophile producing a cyclic pentadienyl cation. Loss of the proton from the carbon bearing the electrophile restores

two electrons to the pi system and restores aroaticity.15-9 Halogenation of Benzene: The Need for a Catalyst

A Lewis acid is required to activated molecular halogens for reaction with benzene. Thus, FeBr2 + Br2 results in

bromination of benzene and AlCl3 + Cl2 results in chlorination. A halogen substituent on benzene reduces the reactivity

and thus further halogenation is slower than the first.

15-10 Nitration and Sulfonation of Benzene

The combination of HNO3 and H2SO4 produces the nitronium ion [O=N=O]+ which reacts with benzene to produce

nitrobenzene. Fuming sulfuric acid, a combination of SO3 and H2SO4, results in sulfonation, producing benzene sulfonic


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acid. Both the nitro and sulfonic acid groups are strongly deactivating and thus further reaction is much slower than is

the first.

15-11 Friedel-Crafts Alkylations

Reaction of benzene with an alkyl halide in the presence of AlCl3 results in alkylation. With primary alkyl halides, the

reactive species is a complex between the alkyl halide and the Lewis acid whereas with secondary and tertiary alkyl

halides, this same complex produces secondary and tertiary cations which then react with benzene.

15-12 Limitations of Friedel-Crafts Alkylations

The presence of an alkyl group increases reactivity and thus alkylation is difficult to control. Further, rearrangements arecommon.

15-13 Friedel-Crafts Alkanoylations (Acylation)

Carboxylic chlorides react with AlCl3 to form a complex very similar to that derived from alkyl chlorides. The CCl bond

in this complex breaks, producing an acylium ion. The acylium ion is stabilized by resonance and therefore does not

undergo rearrangement. Acylium ions react with benzene to produce ketones. The ketone products react irreversibly

with the Lewis acid producing a complex that is ultimately broken by the addition of water. Because the initially formed

ketone reacts with the Lewis acid, the acid must be used in equimolar amounts and is not a catalyst.


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CHAPTER 16 Electrophilic Attack on Derivatives of Benzene: Substituents Control Regioselectivity

16-1 Activation or Deactivation by Substituents on a Benzene Ring

Substituent on an aromatic ring affect the stability of the intermediate pentadienyl cation. As the formation of this

cation is endothermic, the transitition state and therefore the rate of its formation are significantly affected the

substitutent. Those substituents that stabilize carbocations will accelerate the rate of substitution compared with a

hydrogen.

16-2 Directing Inductive Effects of Alkyl Groups

The effect of the substituent on the intermediate pentadienyl cation will vary with its position on this cation: the twoend and the middle carbons bear positive charge whereas the other two do not. Substituents with lone-pairs of

electrons can stabilize the cation by lone pair donation but only when they are on the end or middle carbons. All such

substituents destabilize the cation when on the other two carbons. Groups that are electron withdrawing direct

substitution so that they are on a carbon of the pentadienyl cation that does NOT bear positive charge.

16-3 Directing Effects of Substituents in Conjugation with the Benzene Ring

Substituents with lone-pairs of electrons can stabilize the cation by lone pair donation but only when they are on the

end or middle carbons. All such substituents destabilize the cation when on the other two carbons.

16-4 Electrophilic Attack on Disubstituted Benzenes

When two substituents are present, it is possible that they direct to the same positions. When this is not the case, the

one that is higher on the list of substituent rates (fastest at the top) directs the third group.16-5 Synthetic Strategies Toward Substituted Benzenes

Key here is that there are only five groups that can be directly introduced by electrophilic, aromatic substitution: alkyl,

acyl, halogen, nitro, sulfonic acid. Other groups, such as NH2 and OH are the result of modification of one of the five

groups introduced by EAS

16-6 Reactivity of Polycyclic Benzenoid Hydrocarbons

Naphthalene undergoes electrophilic attach in the same way that benzene does. Substitution at the alpha carbon is

favored.

16-7 Polycylic Aromatic Hydrocarbons and Cancer

In the right concentration, everything will kill you. All peanut butter contains trace amounts of aflatoxins produced by a

fungus that love peanuts as much as people do. Antroplogists were studying a tribe in africa that had an unusually high

level of liver cancer. Yep, peanuts were a major part of their diet. Aflotoxin B1 is shown below. How many functional

groups do you see?


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CHAPTER 17 Aldehydes and Ketones: The Carbonyl Group

17-1 Naming the Aldehydes and Ketones

17-2 Structure of the Carbonyl Group

The carbonyl group has unique structural and reactivity properties by virture of the polarization of (mainly) the pi system

toward the more electronegative oxygen. This imparts electrophilic character to the carbon and nucleophilic character

to the oxygen.

17-3 Spectroscopic Properties of Aldehydes and Ketones

17-4 Preparation of Aldehydes and Ketones

A number of methods for the preparation of ketones and aldehydes have been introduced in previous chapters. They

include:

1. Oxidation of alcohols with Cr+6. PCC must be used with primary alcohols in order that the oxidation does notproceed to the carboxylic acid;

2. Hydration of either a symmetrical or a terminal alkyne (other alkynes result in mixtures of ketones).3. Ozonolysis of an alkene.4. Friedel-Crafts acylation of an aromatic compound.

17-5 Reactivity of the Carbonyl Group: Mechanisms of Additions

There are typically two pathways for addition to carbonyl groups:1. Activation of the carbonyl group by addition of either a proton or a Lewis acid to the oxygen;2. preparation of a reagent with high nucleophilicity.

17-6 Addition of Water to Form Hydrates

Carbonyl groups undergo rapid and reversible reaction with water under both acidic and basic conditions. Except for the

special case of formaldehyde (and some special ketones and aldehydes bearing electron withdrawing groups), the

equilibrium favors the carbonyl compound.

17-7 Addition of Alcohols to Form Hemiacetals and Acetals

17-8 Acetals as Protecting Groups

The typical reactions of carbonyl compounds involve addition to the carbonyl carbon with loss of the pi bond. The

conversion of ketones and aldehydes to ketals and acetals "hides" this reactivity as in these derivatives the pi bond is no

longer present. Because formation of ketals and acetals is reversible, these derivatives can be used to protect the

carbonyl group from reaction.

17-9 Nucleophilic Addition of Ammonia and Its Deriviatives

17-10 Deoxygenation of the Carbonyl Group

17-11 Addition of Hydrogen Cyanide to Give Cyanohydrins

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17-12 Addition of Phosphorus Ylides: The Wittig Reaction

17-13 Oxidation by Peroxycarboxylic Acids: The Baeyer-Villiger Oxidation

17-14 Oxidative Chemical Tests for Aldehydes

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Chem 140b Notes

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