Hydrocarbons

Alkanes, Cycloalkanes, Alkenes Alkynes and Aromatics

Hydrocarbons

Hydrocarbons - compounds containing only hydrogen and carbon

obtained by physical separation from petroleum and natural gas from oil and gas refining Refining - separates complex mixtures into purified components

Petroleum - is a mixture of hundreds of thousands of compounds having boiling points range from 20oC to more than

400oC the difference in boiling points allows the separation of these

compounds by fractionation or fractional distillation

Fractional distillation tower on page 296

Reactions of Hydrocarbons

Cracking – larger hydrocarbons are broken into smaller fragments in the absence of air

Example: C17H36 (l) C9H20 (l) + C8H16 (l)

Reforming – smaller hydrocarbons combine to form larger ones; makes higher grade from lower grade gasoline

Example: C5H12 (l) + C5H12 (l) C10H22 (l) + H2 (g)

Combustion – hydrocarbons burn in the presence of oxygen to form carbon dioxide and water Example: C3H8 (g) + 5 O2 (l) 3 CO2 (l) + 4 H2O (g)

Alkanes

Alkanes – are hydrocarbons with only single C-C bonds – general formula: CnH2n+2

NAMING STRAIGHT CHAIN ALKANES Prefix+ ane prefix = indicates the # of C atoms IUPAC Name Formula methane CH4 (g) ethane C2H6 (g)

propane C3H8 (g) butane C4H10 (g)

pentane C5H12 (l) hexane C6H14 (l) heptane C7H16 (l) octane C8H18 (l) nonane C9H20 (l) decane C10H22 (l)

Naming Branched Chain Alkanes Branch - carbon atoms that are not part of the

parent (longest) chain of carbon atoms - name: prefixyl

Examples: Name Branch methyl -CH3 ethyl -C2H5 (or CH2-CH3) propyl -C3H7 (or CH2-CH2-CH3)

NAMING BRANCHED CHAIN ALKANES Step 1: Identify the longest continuous chain of carbon

atoms (the parent chain) and number the carbon atoms, starting from the end closest to the branch(es).

When there are two chains of equal length, choose the chain with the greater number of branches

When branching first occurs at an equal distance from

either end, number the carbons to give the lowest number at the first point of difference

Step 2: Identify any branches and their location number on the parent chain. Give each branch a number corresponding to its location on the longest chain.

Numbers are separated from words by a hyphen Branches should be listed in alphabetical order When two or more branches are identical:use the

prefixes di, tri, tetra, penta, etc. use commas to separate #s from each other

Step 3: Write the IUPAC name as follows: (# of location) - (branch name) (parent chain)

Parent chain = prefix + “ane” Branch name = prefix + “yl” # of location indicates C on parent chain that the

branch is attached to

*NOTE: use the lowest set of numbers to indicate branch

locations branches are listed in alphabetical order use “,” between #’s when there is more than one

branch use ”-“ to separate #’s from branch names use the prefix “di” “tri” or “tetra” + branch name for

two, three or four identical branches

Example #1: CH3-CH2-CH2-CH2-CH3

Step 1: CH3-CH2-CH2-CH2-CH3

Parent Chain: pentane

Step 2: CH3-CH2-CH2-CH2-CH3

Branch: none

Step 3: (# of location)-(branch name) (parent chain)

NAME: pentane

Example #2: CH3-CH-CH2-CH3

| CH3

Step 1: CH3- CH- CH2- CH3 | CH3

Parent Chain: butane

Step 2: CH3- CH- CH2- CH3 | CH3

Branch: 2-methyl

Step 3: (# of location)-(branch name) (parent chain)

NAME: 2-methylbutane

Example #3: CH3 | CH3- C- CH3

| CH3

Step 1: CH3 | CH3- C- CH3 l CH3

Parent Chain: propane

Step 2: CH3 | CH3- C- CH3 | CH3

Branches: 2,2-dimethyl

Step 3: (# of location)-(branch name) (parent chain) NAME: 2,2-dimethylpropane

Drawing Branched Chain Alkanes

Step 1: Draw a straight chain containing the number of atoms in the parent chain, and number the atoms from left to right.

Step 2: Attach all branches to their numbered locations on the parent chain.

Step 3: Add enough H atoms to show that each C has 4 bonds.

Example 1: Draw a condensed structural diagram for the following:

4-propylheptane

Step 1: C-C-C-C-C-C-C Parent chain: heptane

Step 2: C-C-C-C-C-C-C Branch: 4-propyl

|

C-C-C

Step 3: CH3-CH2-CH2-CH-CH2-CH2-CH3

|

CH2-CH2-CH3

Example 2: Draw a condensed structural diagram for the following:

2,4-dimethylpentane

Step 1: C-C-C-C-C Parent chain: pentane

Step 2: C-C-C-C-C Branches: 2,4-dimethyl

| |

C C

Step 3: CH3-CH-CH2-CH-CH3

| |

CH3 CH3

Example 3: Draw a condensed structural diagram for the following:

3-ethyl-2,4-dimethylhexane

Step 1: C-C-C-C-C-C Parent chain: hexane C-C | Step 2: C-C-C-C-C-C Branches: 3-ethyl & | | 2,4-dimethyl C C Step 3: CH2-CH3

| CH3-CH-CH-CH-CH2-CH3

| | CH3 CH3

Cycloalkanes

cycloalkanes: - are hydrocarbons that form a closed ring - all C-C bonds are single bonds - general formula: CnH2n

Naming: simply add cyclo in front of the alkane name

Examples: cyclopropane: CH2 / \ OR CH2-CH2

cyclobutane: CH2-CH2 | | OR CH2-CH2

cyclopentane: CH2

/ \ CH2 CH2 OR | | CH2 - CH2

Alkenes and Alkynes Alkenes - hydrocarbons with one or more

double carbon-carbon bonds (C=C) named with the suffix “ene” (table 9.5 page 303)

general form: CnH2n

example: CH2=CH2 name: ethene

Alkynes - hydrocarbons with one or more triple carbon-carbon bonds (C=C) named with the suffix “yne” (table 9.6 page 303)

general form: CnH2n-2

Example: CH=CH name: ethyne

Naming Alkenes and Alkynes

Named the same as alkanes with these additional considerations:

the longest chain must contain the multiple bond

the chain is numbered from the end closest to the multiple bond

the parent chain is preceded by a number indicating the position of the multiple bond

Examples: 1.) CH2=CH-CH2-CH3

name: 1-butene 2.) CH3-CH=CH-CH3

name: 2-butene 3.) CH3

| CH3-C=C-CH-CH3

name: 4-methyl-2-pentyne

Saturated versus Unsaturated

Hydrocarbons

Where have you heard the term “saturated” before? Saturated Hydrocarbons - are hydrocarbons that

contain only single bonds and therefore have the maximum number of hydrogen atoms possible (i.e. alkanes)

Example: CH3-CH2-CH3

Unsaturated Hydrocarbons - are hydrocarbons that contain at least one ring or multiple bond and have less than the maximum number of hydrogen atoms possible (i.e. cycloalkanes, alkenes and alkynes)

Example: CH3-CH=CH2

Reactions of Alkenes and Alkynes Unsaturated hydrocarbons like alkenes and alkynes can be

converted to alkanes through a chemical reaction called addition, or hydrogenation.

To convert an alkene to an alkane, one hydrogen molecule for every

alkene molecule is required:

CH3 CH3 l l CH3-C=CH2 + H-H CH3-CH-CH3

2-methyl-1-propene hydrogen 2-methylpropane

To convert and alkyne to an alkane, two hydrogen molecules for

every alkyne molecule are required:

CH3-C=CH + 2 H-H CH3-CH2-CH3

(1-)propyne hydrogen propane

Fats and Food

Saturated Fat – contains all single bonds

Monounsaturated Fat - contains one multiple bond

Polyunsaturated Fat - contains more than one multiple bond

What about “hydrogenated” vegetable oils? (saturated)

What are so called “good fats”?

(unsaturated or non-hydrogenated)

Aromatic Hydrocarbons

Aromatic Compounds

Historically: compounds with an odor or aroma

Today: benzene and all other carbon compounds that have benzene-like structures and properties

Benzene’s possible structure intrigued chemists for years, because its properties could not be explained by accepted theories of bonding and reactivity…

Aromatic Hydrocarbons

Properties of benzene

Molecular Formula: C6H6

no evidence of double or triple bonds

all C-C bonds are the same length

all carbons are identical and each one is bonded to one hydrogen

it is very stable

Aromatic Hydrocarbons

August Kekule proposed a cyclic structure for benzene

(1865) inspired by a dream of a snake who grabbed its own tail

disputed because evidence shows all C-C bonds

are the same length, and neither a single or

double bond is present

Aromatic Hydrocarbons

Current benzene model:

based on the resonance theory and the molecular orbital theory)

structure is a hybrid of the 2 structures shown by Kekule

valence electrons are evenly distributed (delocalized) over the entire ring

Naming Aromatics

named as relatives of benzene

One substituent: “alkyl” benzene

CH3 CH2CH3

methylbenzene ethylbenzene

Naming Aromatics

Two substituents: must indicate the location of the substituents

Numbering starts at one of the substituents and goes counterclockwise or clockwise to obtain the lowest possible pair of numbers

CH3 CH3 CH3

CH3

CH3

CH3

1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene

ortho meta para

o-dimethylbenzene m-dimethylbenzene p-dimethylbenzene

Naming Aromatics

Larger Molecules: benzene ring is a branch (phenyl group -C6H5)

CH3-CH-CH2-CH2-CH3

2-phenylpentane

Aliphatic vs. Aromatic

Any hydrocarbon in which the bonding capacity of carbon is 4 is considered an aliphatic hydrocarbon All alkanes, cycloalkanes, alkenes and alkynes are

considered aliphatic as the carbon atoms in these molecules all have a bonding capacity of 4.

Any hydrocarbon in which the bonding capacity of carbon is 3 is considered an aromatic hydrocarbon All molecules containing the benzene aromatic ring

are considered aromatic as the carbon atoms in these molecules all have a bonding capacity of 3

Aliphatic or Aromatic ?

A. CH3-CH=CH-CH3

Aliphatic

B. CH2CH3

Aromatic

C. methylpropene

Aliphatic

D. 1,2-dimethylbenzene Aromatic

E. 2-phenylhexane

Aromatic

Reactions of Aromatic

Hydrocarbons Aromatic hydrocarbons can undergo cracking, reforming and combustion

similar to other hydrocarbons

Cracking – larger aromatic hydrocarbons are broken into smaller fragments in the absence of air

CH2-CH2-CH3

Example: + H-H + CH3-CH2-CH3

Reforming – smaller aromatic hydrocarbons combine to form larger ones; CH=CH2

Example: + CH2= CH2 + H-H

Combustion – hydrocarbons burn in the presence of oxygen to form

carbon dioxide and water CH3

Example: + 9 O2 7 CO2 + 4 H2O