Chapter 4 Introduction to Hydrocabons

Chapter 4

Introduction toHydrocabons

Carbon Backbone, Nomenclature, Physical &

Chemical Properties

HYDROCARBONS• Compounds composed of only carbon and hydrogen atoms

(C, H). Each carbon has 4 bonds.

• They represent a “backbone” when other “heteroatoms” (O, N, S, .....) are substituted for H. (The heteroatoms give function to the molecule.)

• Acyclic (without rings); Cyclic (with rings); Saturated: only carbon-carbon single bonds; Unsaturated: contains one or more carbon-carbon double and/or triple bond

HYDROCARBONS• Alkanes contain only single ( ) bonds and have the

generic molecular formula: [CnH2n+2]

• Alkenes also contain double ( + ) bonds and have the generic molecular formula: [CnH2n]

• Alkynes contain triple ( + 2) bonds and have the generic molecular formula: [CnH2n-2]

• Aromatics are planar, ring structures with alternating single and double bonds: eg. C6H6

Types of Hydrocarbons

Each C atom is trigonal planar with sp2 hybridized orbitals.There is no rotation about the C=C bond in alkenes.

Each C atom is tetrahedral with sp3 hybridized orbitals. They only have single bonds.

Question 4.1

• What is the hybridization of the starred carbon in humulene (shown)?

• A) sp• B) sp2

• C) sp3

• D) 1s2 2s2 2p2

Question 4.2

• What is the hybridization of the starred carbon of geraniol?

• A) sp• B) sp2

• C) sp3

• D) 1s2 2s2 2p2

Types of Hydrocarbons

Each C atom is linear with sp hybridized orbitals.

Each C--C bond is the same length; shorter than a C-C bond: longer than a C=C bond.The concept of resonance is used to explain this phenomena.

Propane

It is easy to rotate about the C-C bond in alkanes.

Naming AlkanesC1 - C10 : the number of C atoms present in the chain.

Each member C3 - C10 differs by one CH2 unit. This is called a homologous series.Methane to butane are gases at normal pressures.Pentane to decane are liquids at normal pressures.

Nomenclature of Alkyl Substituents

Examples of Alkyl Substituents

Constitutional or structural isomers have the same molecular formula, but their atoms are linked differently. Naming has to account for them.

Question 4.3

• How many hydrogens are in a molecule of isobutane?

• A) 6• B) 8• C) 10• D) 12

A compound can have more than one name, but a name must unambiguously specify only one compound

C7H16 can be any one of the following:

Question 4.4

• How many isomeric hexanes exist?• A) 2• B) 3• C) 5• D) 6

Question 4.5• The carbon skeleton shown at the bottom right

accounts for 9 carbon atoms. How many other isomers of C10H22 that have 7 carbons in their longest continuous chain can be generated by adding a single carbon to various positions on this skeleton?

• A) 2• B) 3• C) 4• D) 5

Alkanes (Different types of sp3 carbon atoms)• Primary, 1o, a carbon atom with 3 hydrogen atoms:

[R-CH3]

• Secondary, 2o, a carbon atom with 2 hydrogen atoms:

[R-CH2-R]

• Tertiary, 3o, a carbon atom with 1 hydrogen atom:

• [R-CH-R] R

• Quaternary, 4o, a carbon atom with 0 hydrogen atoms: CR4

Different Kinds of sp3 Carbons and Hydrogens

Question 4.6• In 3-ethyl-2-methylpentane, carbon #3

(marked by a star) is classified as:

• A) primary (1°)• B) secondary (2°)• C) tertiary (3°)• D) quaternary (4°)

Question 4.7

• How many primary carbons are in the molecule shown at the bottom right?

• A) 2• B) 3• C) 4• D) 5

Nomenclature of Alkanes1. Determine the number of carbons in the parent hydrocarbon

CH3CH2CH2CH2CHCH2CH2CH3

CH3

12345678CH3CH2CH2CH2CHCH2CH3

CH2CH2CH3

45678

123

CH3CH2CH2CHCH2CH2CH3

CH2CH2CH2CH3

1234

5 6 7 8

2. Number the chain so that the substituent gets the lowest possible number

CH3CHCH2CH2CH3

CH3

1 2 3 4 5

2-methylpentane

CH3CH2CH2CHCH2CH2CH2CH3

CHCH3

CH3

1 2 3 4 5 6 7 8

4-isopropyloctane

CH3CHCH2CH2CH3

CH3

common name: isohexanesystematic name: 2-methylpentane

3. Number the substituents to yield the lowest possible number in the number of the compound

CH3CH2CHCH2CHCH2CH2CH3

CH3 CH2CH3 5-ehtyl-3-methyloctanenot

4-ethyl-6-methyloctanebecause 3<4

(substituents are listed in alphabetical order)

4. Assign the lowest possible numbers to all of the substituents

CH3CH2CHCH2CHCH3

CH3 CH3

2,4-dimethylhexane

CH3CH2CH2C

CH3

CH3

CCH 2CH 3

CH3

CH3

3,3,4,4-tetramethylheptane

CH3CH2CHCH2CH2CHCHCH2CH2CH3

CH2CH3

CH2CH3 CH2CH3

CH3

3,3,6-triethyl-7-methyldecane

5. When both directions lead to the same lowest number for oneof the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents

CH3CHCH2CHCH3

CH3

CH3 CH3

2,2,4-trimethylpentanenot

2,4,4-trimethylpentanebecause 2<4

CH3CH2CHCHCH2CHCH2CH3

CH3

CH3 CH2CH3

6-ethyl-3,4-dimethyloctanenot

3-ethyl-5,6-dimethyloctanebecause 4<5

6. If the same number is obtained in both directions, the firstgroup receives the lowest number

CH3CH2CHCH2CHCH2CH3

CH3

CH2CH3

3-ethyl-5-methylheptanenot

5-ethyl-3-methylheptane

CH3CH2CHCH3

Cl

Br

2-bromo-3-chlorobutanenot

3-bromo-2-chlorobutane

7. In the case of two hydrocarbon chains with the same number ofcarbons, choose the one with the most substituents

CH3CH2CHCH2CH2CH3

CHCH3

CH31

2

3 4 5 6

3-ethyl-2-methylhexane (two substituents)

CH3CH2CHCH2CH2CH3

CHCH3

CH3

1 2 3 4 5 6

3-isopropylhexane (one substituent)

8. Certain common nomenclatures are used in the IUPAC system

CH3CH2CH2CH2CHCH2CH2CH3

CHCH3

CH3

4-isopropyloctaneor

4-(1-methylethyl)octane

CH3CH2CH2CH2CHCH2CH2CH2CH2CH3

CH2CHCH3

CH3

5-isobutyldecaneor

5-(2-methylpropyl)decane

Question 4.7

• The correct structure of 3-ethyl-2-methylpentane is:

• A) B)

• C) D)

CnH2n

Cycloalkane Nomenclature

Cycloalkanes• Cycloalkanes are alkanes that contain a

ring of three or more carbons.• Count the number of carbons in the ring,

and add the prefix cyclo to the IUPAC name of the unbranched alkane that has that number of carbons.

Cyclopentane Cyclohexane

Ethylcyclopentane

CH2CH3

• Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent.

Cycloalkanes

• Name any alkyl groups on the ring in the usual way. A number is not needed for a single substituent.

• List substituents in alphabetical order and count in the direction that gives the lowest numerical locant at the first point of difference.

3-Ethyl-1,1-dimethylcyclohexane

CH2CH3

H3C CH3

Cycloalkanes

For more than two substituents,

CH3CH2CH2

H3C CH2CH3

4-ethyl-2-methyl-1-propylcyclohexanenot

1-ethyl-3-methyl-4-propylcyclohexanebecause2<3

not 5-ethyl-1-methyl-2-propylcyclohexane

because 4<5

CH3

CH3

CH3

1,1,2-trimethylcyclopentanenot

1,2,2-trimethylcyclopentanebecause1<2

not1,1,5-trimethylcyclopentane

because 2<5

Question 4.8

• Which one contains the greatest number of tertiary carbons?

• A) 2,2-dimethylpropane• B) 3-ethylpentane• C) sec-butylcyclohexane• D) 2,2,5-trimethylhexane

Physical Properties of Alkanes

and Cycloalkanes

Crude oil

Refinery gas

C1-C4

Light gasoline(bp: 25-95 °C)

C5-C12

Naphtha(bp 95-150 °C)

Kerosene(bp: 150-230 °C)

C12-C15

Gas oil(bp: 230-340 °C)

C15-C25

Residue

Question 4.9Arrange octane, 2,2,3,3-tetramethylbutane and

2-methylheptane in order of increasing boiling point.

• A) 2,2,3,3-tetramethylbutane < octane < 2-methylheptane

• B) octane < 2-methylheptane < 2,2,3,3-tetramethylbutane

• C) 2,2,3,3-tetramethylbutane < 2-methylheptane < octane

• D) 2-methylheptane < 2,2,3,3-tetramethylbutane < octane

• The gasoline fraction of crude oil only makes up about 19%, which is not enough to meet demand.

Crude Oil and Uses of Alkanes

van der Waals ForcesWeak Intermolecular Attractive Forces

The boiling point of a compound increases with the increase in van der Waals force…and a

Gecko uses them to walk!

Gecko: toe, setae, spatulae6000x Magnification

http://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html

Geim, Nature Materials (2003) Glue-free Adhesive100 x 10 6 hairs/cm2

Full et. al., Nature (2000)5,000 setae / mm2

600x frictional force; 10-7 Newtons per seta

Ion-Dipole Forces (40-600 kJ/mol)• Interaction between an ion and a dipole (e.g. NaOH and

water = 44 kJ/mol)• Strongest of all intermolecular forces.

Intermolecular Forces

Ion-Dipole & Dipole-Dipole Interactions: like dissolves like

• Polar compounds dissolve in polar solvents & non-polar in non-polar

Dipole-Dipole Forces(permanent dipoles)

Intermolecular Forces

5-25 kJ/mol

Dipole-Dipole Forces

Intermolecular Forces

Boiling Points &Hydrogen Bonding

Hydrogen Bonding

• Hydrogen bonds, a unique dipole-dipole (10-40 kJ/mol).

London or Dispersion Forces• An instantaneous dipole can induce another dipole in an

adjacent molecule (or atom).• The forces between instantaneous dipoles are called

London or Dispersion forces ( 0.05-40 kJ/mol).

Intermolecular Forces

Boiling Points of Alkanes

• governed by strength of intermolecular attractive forces

• alkanes are nonpolar, so dipole-dipole and dipole-induced dipole forces are absent

• only forces of intermolecular attraction are induced dipole-induced dipole forces

Boiling Points• Increase with increasing number of carbons• more atoms, more electrons, more

opportunities for induced dipole-induceddipole forces

• Decrease with chain branching• branched molecules are more compact with

smaller surface area—fewer points of contactwith other molecules

London Dispersion Forces

Intermolecular Forces

Which has the higherattractive force?

Question 4.10

• Which alkane has the highest boiling point?

• A) hexane• B) 2,2-dimethylbutane• C) 2-methylpentane• D) 2,3-dimethylbutane

•Increase with increasing number of carbons

• more atoms, more electrons, more opportunities for induced dipole-induceddipole forces

Heptanebp 98°C

Octanebp 125°C

Nonanebp 150°C

Boiling Points

•Decrease with chain branching• branched molecules are more compact with

smaller surface area—fewer points of contact

with other molecules Octane: bp 125°C

2-Methylheptane: bp 118°C

2,2,3,3-Tetramethylbutane: bp 107°C

Boiling Points

• Gasoline is a mixture of straight, branched, and aromatic hydrocarbons (5–12 carbons in size).

– Large alkanes can be broken down into smaller molecules by CRACKING.

– Straight chain alkanes can be converted into branched alkanes and aromatic compounds through REFORMING.

– After using these processes, the yield of gasoline is about 47% rather than 19%.

Sources and Uses of Alkanes

•All alkanes burn in air to givecarbon dioxide and water.

Chemical Properties:Combustion of Alkanes

4817 kJ/mol

5471 kJ/mol

6125 kJ/mol

654 kJ/mol

654 kJ/mol

Heptane

Octane

Nonane

Heats of Combustion

What pattern is noticed in this case?

•Increase with increasing number of carbons• more moles of O2 consumed, more moles

of CO2 and H2O formed

Heats of Combustion

5471 kJ/mol

5466 kJ/mol

5458 kJ/mol

5452 kJ/mol

5 kJ/mol

8 kJ/mol

6 kJ/mol

Heats of Combustion

What pattern is noticed in this case?

8CO2 + 9H2O

5452 kJ/mol5458 kJ/mol

5471 kJ/mol

5466 kJ/molO2+ 25

2

O2+ 252 O2+ 25

2 O2+ 252

ENERGY Diagrams /

Reaction Coordinate Diagrams

•Increase with increasing number of carbons• more moles of O2 consumed, more moles

of CO2 and H2O formed

•Decrease with chain branching• branched molecules are more stable

(have less potential energy) than theirunbranched isomers

Heat of CombustionPatterns

•Isomers can differ in respect to their stability.

•Equivalent statement:

–Isomers differ in respect to their potential energy.

Important Point

Differences in potential energy can be measured by comparing heats of combustion. (Worksheet problems)