Unit 2 – Alkanes and Chemical Reactions

Unit 2 – Alkanes and Chemical Reactions

Structure and Stereochemistry of Alkanes Nomenclature of alkanes and

cycloalkanes Physical Properties Conformational Analysis

The Study of Chemical Reactions Kinetics and Thermodynamic Quantities Free Radical Halogenation Reactive Intermediates and Transition

States

Hydrocarbons The simplest organic compounds are the

hydrocarbons: organic compounds that contain only

carbon and hydrogen

four general types: alkanes alkenes alkynes aromatic hydrocarbons

Hydrocarbons Alkanes are often called saturated

hydrocarbons they contain the maximum number of

hydrogen atoms per carbon atom.

Alkenes, alkynes, and aromatic hydrocarbons are called unsaturated hydrocarbons they contain fewer H atoms than an

alkane with the same number of carbon atomsH C CH

HH

H

HH C C HC C

H

H

H

H

Alkanes You must know the names and formulas

for the 10 simplest alkanes:

CH4 methaneCH3CH3 ethaneCH3CH2CH3 propaneCH3CH2CH2CH3 butaneCH3CH2CH2CH2CH3 pentaneCH3CH2CH2CH2CH2CH3 hexane

Alkanes You must know the names and

formulas for the 10 simplest alkanes:

CH3CH2CH2CH2CH2CH2CH3 heptaneCH3CH2CH2CH2CH2CH2CH2CH3 octaneCH3CH2CH2CH2CH2CH2CH2CH2CH3 nonaneCH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 decane

Alkanes The alkanes form a homologous series

with a general molecular formula of CnH(2n+2)

Homologous series: a series of compounds in which each

member differs from the next member by a constant unit

Alkanes differ from each other by -CH2-

AlkanesExample: Which of the following are alkanes:

C2H6, C3H6, C5H12, C4H8

Example: What is the formula for an alkane with 12 carbons?

Alkanes The previous alkanes are straight-chain

alkanes: all of the carbon atoms are joined in a

continuous chain also called “normal” alkanes (n-alkanes)

Alkanes containing 4 or more carbons can also form branched alkanes one or more of the carbon atoms form a

“branch” or side-chain off of the main chain

Alkanes An example of a straight chain alkane:

C5H12 pentane

Examples of branched alkanes:

C5H12

2-methylbutane

2,2-dimethylpropane

CH3CHCH2CH3

CH3

CH3CH2CH2CH2CH3

CH3CCH3

CH3

CH3

Alkanes The three structures shown previously for

C5H12 are structural (constitutional) isomers: compounds with the same molecular

formula but different bonding arrangements

Structural isomers have different properties: different melting points different boiling points often different chemical reactivity

Alkane Nomenclature Organic compounds can be named either

using common names or IUPAC names.

CH3CH2CH2CH2CH3

CH3CHCH2CH3

CH3CCH3

CH3

CH3

CH3

CH3CH2CH2CH2CH3

CH3CHCH2CH3

CH3CCH3

CH3

CH3

CH3

CH3CH2CH2CH2CH3

CH3CHCH2CH3

CH3CCH3

CH3

CH3

CH3

pentane

2-methylbutane or

isopentane

2,2-dimethylpropane or

neopentane

Alkane Nomenclature

CH3 CHCH3

R

The common name for any alkane containing a CH3 group on the second carbon in the chain is “isoalkane.”

“iso”

Isohexane (6 C total)

Isobutane

(4 C total)

CH3CHCH3

CH2CH2CH3

CH3CHCH3

CH3

Alkane Nomenclature Most of the time, organic chemists use

the IUPAC names for organic compounds.

LEARN THE RULES FOR EACH CLASS OF COMPOUNDS WE DISCUSS.

Alkane Nomenclature To name an alkane:

Find the longest continuous chain of carbon atoms and use the name of that chain as the base name of the compound: the longest chain is often NOT written in a straight line

Base name: Base name: heptaneheptaneCH3CHCH2CH3

CH2CH2CH2CH3

123

4 5 6 7

Alkane Nomenclature Number the carbon atoms in the longest

chain starting at the end of the chain closest to a substituent a group attached to the main chain that

has taken the place of a hydrogen atom on the main chain

A substituentA substituent CH3CHCH2CH3CH2CH2CH2CH3

123

4 5 6 7

Alkane Nomenclature Name and give the location of each

substituent. Common substituents:

Halo group a halogen atom “Halo” groups are named using “halo”:

Cl chloro Br bromo I iodo F fluoro

Nitro group -NO2

Alkane Nomenclature Common substituents:

alkyl group A group that is formed by removing an H atom from an alkane

the alkyl group attaches to the main chain at the carbon that has lost its H

CH3CH2CHCH2CH2CH3CH2CH3

correct

CH3CH2CHCH2CH2CH3CH3CH2

incorrect

Alkane Nomenclature Alkyl groups are named by replacing the “ane”

ending of the parent alkane with the “yl” ending.

CH4 H CH

Hmethyl

CH3CH2 ethylCH3CH3

CH3CH2CH3

CH3CH2CH2 propyl

CH3CHCH3 isopropyl

Alkane Nomenclature

CH3CH2CH2CH3

CH3CH2CH2CH2 butyl

CH3CH2CHCH3 sec- butylbutane

CH3CHCH3

CH3

CH3CHCH2

CH3

CH3CCH3

CH3

isobutyl

t- butylisobutane

Alkane Nomenclature

CH3CCH3

CH3

CH3

CH3CCH2

CH3

CH3

neopentyl

neopentane

phenyl

benzene

Alkane Nomenclature Complex alkyl substituents

Use the longest alkyl chain of the substituent as the base name of the substituent

Number the substituent chain with the “head carbon” as carbon 1

List substituents on the alkyl chain with the appropriate numbers

Use parentheses around the name of the group

CHCHCH3

CH2CH3

CH3

a (1- ethyl- 2- methylpropyl) group

Alkane Nomenclature

Methyl groupMethyl group

3-methylheptane

Note: Separate numbers from letters using a hyphen. Separate groups of numbers using commas.

CH3CHCH2CH3CH2CH2CH2CH3

123

4 5 6 7

Alkanes Alkane Nomenclature:

When two or more substituents are present, list them in alphabetical order:

isopropyl before methyl t-butyl or sec-butyl before chloro

When more than one of the same substituent is present (i.e. two methyl groups), use prefixes to indicate how many. Give the location of each as well.

Di = two Tri = three Tetra = four Penta = five Hexa = six

Know Know these.these.Note: Ignore these prefixes when alphabetizing.

Alkane Nomenclature Additional rules:

When there are two “longest” chains of equal length, use the chain with the greater number of substituents.

incorrect correct

CH3 CH CH2

CH3

CHCH3

CH CH2 CH3

CHCH3

CH3

CH3 CH CH2

CH3

CHCH3

CH CH2 CH3

CHCH3

CH3

Alkane Nomenclature Additional rules:

If each end of the longest chain has a substituent the same distance from the end, start with the end nearer to the second substituent.

correct incorrect3-chloro-2,5-dimethylhexane

CH3CHCHCH2CHCH3

CH3

Cl CH31 2

3 4 5 6CH3CHCHCH2CHCH3

CH3

Cl CH3

1234

5

6

AlkanesExample: Name the following compounds:

CH3CHCHCH3CH2CH3

Br

AlkanesExample: Name the following compound:

CH3CHCHCH2CHCH3C(CH3)3

CH(CH3)2

CH2CH3

CH3CCH2CH2NO2CH2CH3

CHCH3

CH3

Alkanes You must also be able to write the structure

of an alkane when given the IUPAC name.

Identify the main chain and draw the carbons in it.

Identify the substituents (type and #) and attach them to the appropriate carbon atoms on the main chain.

Add hydrogen atoms to the carbons to make a total of 4 bonds to each carbon

AlkanesExample: Draw the structure for the following compounds:

3, 3-dimethylpentane

4-sec-butyl-2-methyloctane

1,2-dichloro-3-methylheptane

2-nitropropane

Alkane NomenclatureExample: Draw the structural isomers of hexane (C6H14). Name each isomer.

Use a systematic approach to draw structural isomers:

Draw the unbranched isomer for the first structure.

For other structures, remove 1 or more carbons (and/or functional groups) from the unbranched isomer and reposition to make unique compounds

Types of Carbon Atoms Primary carbon (1o)

a carbon bonded to one other carbon

Secondary carbon (2o) a carbon bonded to two other carbons

Tertiary carbon (3o) a carbon bonded to three other carbons

C

C

C

CH

HH

C CH

HCCC

H

C

C

C

CH

HH

C CH

HCCC

H

C

C

C

CH

HH

C CH

HCCC

H

C

C

C

CH

HH

C CH

HCCC

H

Physical Properties Solubility

Alkanes are nonpolar hydrophobic

do not dissolve in water soluble in nonpolar or weakly polar organic solvents

Density: varies from ~0.5 - ~0.8 g/mL

less dense than water (1.0 g/mL) Alkanes float on water

Physical Properties Boiling Point

In general, boiling point increases as the molecular weight of the alkane increases

larger molecules have greater surface area and higher London dispersion forces of attraction

must be overcome for vaporization and boiling to occur

Physical Properties Boiling Point (cont)

Given the same total number of carbon atoms:

BP (branched) < BP (n-alkane)

Branched alkanes are more compact. less surface area

smaller London dispersion forces lower BP

Physical Properties Melting Points:

In general, melting point increases as MW increases

irregular, sawtooth pattern

Physical Properties Melting Point:

Alkanes with odd number of carbons have lower than expected melting points (compared to the previous alkane with an even number of carbons)

Even # carbons better packing in solid structure

higher MP Odd # carbons

do not pack as well lower MP

Physical Properties Melting Points:

Given the same total number of carbon atoms:

MP (branched) > MP (n-alkane) branched alkanes have more compact structure

better packing higher MP

Sources & Uses of Alkanes Alkanes are derived primarily from

petroleum and petroleum by-products:

Refining via fractional distillation gives useful mixtures of alkanes: C2 - C4 liquified petroleum

gas C4 - C9 gasoline C8 - C16 kerosene C10 - C18 diesel C16+ heavy/mineral oil

Reactions of Alkanes Catalytic Cracking:

converts alkanes into more valuable mixtures of smaller alkanes and alkenes

alkane smaller alkanes + alkenes

C12H26

SiO2 or Al2O3

catalyst

SiO2 +

Reactions of Alkanes Hydrocracking:

converts higher alkanes into shorter alkanes and eliminates N and S impurities

Alkane shorter alkanes

C12H26

H2, SiO2 +

H2, SiO2 or Al2O3

catalyst

Reactions of Alkanes Combustion:

a rapid, exothermic redox reaction that converts hydrocarbons into carbon dioxide and water

alkane + O2 CO2 + H2O

2 C6H14 + 19 O2 12 CO2 + 14 H2O

(unbalanced)

Reactions of Alkanes Halogenation:

a reaction in which a halogen atom is substituted for a hydrogen atom on an alkane

alkane + X2 mixture of alkyl halides

CH4 + Cl2 CH3Cl + CH2Cl2 + CHCl3 + CCl4

or h

h

unbalanced

Conformations of Alkanes•The simplest alkane, CH4, is perfectly

tetrahedral:•bond angle = 109.5•C-H bond length = 1.09 A•free rotation of the C-H bond

Conformations of Alkanes Ethane:

Two carbons overlapping sp3 hybrid orbitals form a sigma bond

Conformations of Alkanes The two methyl groups are relatively free to

rotate about the sigma bond between the two carbon atoms sigma bond maintains its overlap at all times

The different arrangements formed by rotation around a single bond are called conformations.

Conformer: a specific conformation a “conformational isomer”

Conformations of Alkanes Conformers are often drawn using

Newman projections: a way of drawing a molecule looking

straight down the bond connecting two carbon atoms

front carbon atom is represented by three lines joined together in a Y shape

back carbon is represented by a circle with three bonds pointing out from it

H

HHH

HH

Conformations of Alkanes

C

C

HH

HH

H

H

CH

HHH

HH

C

C

HH

HH

H

H

CH

HHH

HH3-D structure

of one conformer of ethane

View from this end

Newman projection

=

Conformations of Alkanes An infinite number of conformations are

possible for ethane and higher alkanes. The dihedral angle () can have an

infinite number of values angle between the C-H bonds on the front and back carbons

C

C

HH

HH

H

H

CH

HHH

HH

Conformations of Alkanes

Molecules are constantly rotating through all possible conformations.

Important conformations for ethane:

Eclipsed conformation

Staggered conformation

Skew conformation

Conformations of Alkanes The conformation of ethane changes

constantly at room temperature.

Conformations may have different energies. Lowest energy conformer is most favored. Highest energy conformer is least favored.

Conformational analysis: the study of the energies of different

conformations helps predict which conformation are

favored and which reaction may occur

Conformations of Alkanes Staggered conformation of ethane:

lowest energy most favored electron clouds in the C - H bonds separated as much as possible

Eclipsed conformation of ethane: highest energy least favored

electron clouds of C - H bonds are closest together

Conformations of Alkanes As ethane rotates from the staggered

conformation towards the eclipsed conformation:

potential energy increases due to torsional strain

resistance to rotation or twisting about a bond

Conformations of Alkanes

Conformation of Alkanes Butane:

4 carbon chain held together by end-to-end overlap of sp3 hybrid orbitals on the carbon atoms

tetrahedral geometry around each carbon

free rotation about any C - C bond many conformers of differing energies

are possible

Newman projections of butane are drawn by looking down the central C2 - C3 bond.

Conformations of Alkanes

Totally eclipsed

(0o)Gauche

(60o)Eclipsed

(120o)Anti

(180o)

Conformation of Alkanes Totally eclipsed conformer of butane:

highest energy due to steric hinderance between the methyl groups

Steric hinderance: interference between two bulky groups

that are close enough together so that their electron clouds repel each other

Conformations of Alkanes

Conformations of Alkanes Eclipsed conformer of butane:

second highest in energy due to repulsion of the methyl group on one carbon and the hydrogen atom on the other

All staggered conformers (gauche and anti) of butane are lower in energy than any of the eclipsed conformers.

Anti conformer of butane: lowest energy because methyl groups are

furthest apart

Conformations of Alkanes