HYDROCARBONS

• Hydrocarbons are composed, as their name suggests mainly of carbon and hydrogen atoms, though some types have oxygen, nitrogen or sulphur atoms.

• Hydrocarbons are divided into two groups, Aliphatics or Aromatics. Most hydrocarbons are aliphatics.

Representing Hydrocarbon

• There are four ways in which hydrocarbons can be represented.

• 1. Chemical Formula We are used to using chemical formulas to represent compounds. Unfortunately, chemical formulas are not useful when dealing with organic molecules because the arrangement of the atoms within the molecule is important. This can be seen with the sugars glucose and fructose.

Comparison of glucose and fructose

• 2. Structural formulas are very useful and important in organic chemistry. A structural formula shows both the type and arrangement of the elements and all bonds.

• 3. Condensed formulas show major bonds but normal C to H bonds are not drawn.

•CH3CH2CH2CH2CH3

• 4. Line diagrams represent the molecule using lines. The end of a line means CH3 while a corner is CH2

ALKANES

• Alkanes area class of HYDROCARBONS which contain only carbon and hydrogen. Two other terms which describe alkanes are saturated and paraffins.

• Alkanes are SATURATED which means that each carbon is bonded to four other atoms through single covalent bonds. Hydrogen atoms usually occupy all available bonding positions after the carbons have bonded to each other.

• PARAFFINS which is derived from a Latin word meaning "little activity", and means that the compounds are very unreactive.

• The general structure is CnH2n+2

Two common alkanes

Properties

Physical Properties• Boiling points depend on

chain length. The longer the chain the higher the boiling temperature

• Non polar• Insoluble in water

Chemical Reactivity• No functional groups,

saturated, and class name of paraffins which means - Unreactive to other chemicals.

• All compounds: Combustion Reaction

ROOT ALKANE NAMES

• Root names give the number of carbons in the longest continuous chain. Root names are used with various "endings" to indicate branches, type of bonds between carbons, and functional groups.

• The following list gives the most basic root the with normal hydrocarbon alkane endings for the number of carbons in the longest continuous chain. Memorize this list.

• Example: root = "eth" + alkane ending = "ane" = ethane

 No. ofCarbons

 Root Name FormulaCnH2n+2

 Structure

 1  methane  CH4  CH4

 2  ethane  C2H6  CH3CH3

 3  propane  C3H8  CH3CH2CH3

 4  butane  C4H10  CH3CH2CH2CH3

 5  pentane   C5H12  CH3CH2CH2CH2CH3

 No. ofCarbons

 Root Name FormulaCnH2n+2

 Structure

 6  hexane  C6H14  CH3CH2CH2CH2CH2CH3

 7  heptane  C7H16  CH3CH2CH2CH2CH2CH2CH3

 8  octane  C8H18  CH3CH2CH2CH2CH2CH2CH2CH3

 9  nonane  C9H20  CH3CH2CH2CH2CH2CH2CH2CH2CH3

 10  decane  C10H22  CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

Timberlake LecturePLUS 1999 13

Cycloalkanes

• Cycloalkanes contain a carbon chain that is in the form of a ring. Each cycloalkane has a formula that is 2H less than the corresponding alkane. For example, propane is C3H8 whereas cyclopropane is C3H6. Butane is C4H10 and cyclobutane is C4H8. The names of the cyclic structures use the prefix cyclo in from of the alkane name for the carbon chain.

Timberlake LecturePLUS 1999 14

CycloalkanesCyclopropane CH2

CH2 CH2

Cyclobutane CH2

CH2

CH2 CH2

Timberlake LecturePLUS 1999 15

More Cycloalkanes

Cyclopentane CH2

CH2 CH2

CH2 CH2

Cyclohexane CH2

CH2 CH2

CH2 CH2

CH2

Timberlake LecturePLUS 1999 16

Naming Cycloalkanes with Side Groups

Number of Namingside groups One Side group name goes in front

of the cycloalkane name. Two Number the ring in the direction

that gives the lowest numbers to the side groups and add the number to the name.

Timberlake LecturePLUS 1999 17

Cycloalkanes with Side Groups

1,2,4-trimethylcyclohexane

1,2-dimethylcyclopentane

methylcyclopentane

CH3

CH3

CH3

CH3

CH3

CH3

ISOMERS

• Isomers are compounds which have the same formula but have different forms. Some isomers are known as Structural Isomers because they have a completely different atomic arrangement, while others are called Geometric Isomers because of the spacial arrangement.

Isomers of C4H10

Butane 2-Methylpropane

Reaction of Alkanes

• Alkanes are very unreactive in most cases. The exceptions are Combustion and Halogenation.

• Alkanes readily react with oxygen in combustion reactions to form carbon dioxide and water. The shorter chain alkanes are the ones which we normally think of as fuel such as Methane (natural gas), propane for bbqs, butane for lighters and octane for the car.

• Halogenation involves the reaction of the alkane with a halogen such as F2, Cl2, Br2, or I2.

• The product is an alkyl halide in which a H is replaced by a halogen atom. If the halogen is in excess the reaction contiunes until all H are replaced.

• CH4 + F2 → CH3F + HF Fluromethane

• CH3F + F2 → CH2F2 + HF Difluoromethane

• CH2F2 + F2 → CHF3 + HF Trifluoromethane

• CHF3 + F2 → CF4 + HF carbon tetrafluoride