9/5/12

1

Chapter 4

Introduction to Hydrocabons

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 (σ ) bonds and have the

generic molecular formula: [CnH2n+2]

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

•  Alkynes contain triple (σ + 2π) 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

9/5/12

2

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.

Nomenclature of Alkyl Substituents Examples of Alkyl Substituents

9/5/12

3

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

9/5/12

4

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 Alkanes 1. Determine the number of carbons in the parent hydrocarbon

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

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

(substituents are listed in alphabetical order)

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

9/5/12

5

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

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

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

8. Certain common nomenclatures are used in the IUPAC system

Question 4.7

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

•  A) B)

•  C) D)

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.

9/5/12

6

•  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.

Cycloalkanes

For more than two substituents, 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

9/5/12

7

Question 4.9 Arrange 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 Forces Weak 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, spatulae 6000x Magnification

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

Geim, Nature Materials (2003) Glue-free Adhesive 100 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.

9/5/12

8

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

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

(permanent dipoles)

5-25 kJ/mol

Dipole-Dipole Forces

Boiling Points & Hydrogen Bonding

Hydrogen Bonding

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

9/5/12

9

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).

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-induced dipole forces

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

smaller surface area—fewer points of contact with other molecules

London Dispersion Forces

Which has the higher!attractive 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-induced dipole forces

Boiling Points

9/5/12

10

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

smaller surface area—fewer points of contact with other molecules

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 give carbon dioxide and water.

Chemical Properties: Combustion of Alkanes

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 Heats of Combustion

What pattern is noticed in this case?

9/5/12

11

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 their unbranched isomers

Heat of Combustion Patterns

• 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)