Unsaturated Hydrocarbons

Double and Triple bonds between Carbons

Objectives

To distinguish between the unsaturated hydrocarbons called alkenes and alkynes and recognize how they differ from alkanes

To draw structural diagrams and write structural formulae of alkenes and alkynes

To name unsaturated hydrocarbons

To identify the molecular geometry of alkenes and alkynes

Review

Alkanes are saturated hydrocarbons

Saturated hydrocarbons have only single bonds

Examples of alkanes include methane, ethane, propane, 2,3-diethyloctane...

We use system of prefixes to name alkanes

Review

Number of Carbons Prefix

1 meth-

2 eth-

3 prop-

4 but-

5 pent-

6 hex-

7 hept-

8 oct-

9 non-

10 dec-

Review

Name based on longest chain of carbons

Use digits to indicate location of carbon attached to substituents

For example: 5-ethyl-3- methylheptane

How to tell from formula if saturated

In a saturated compound, noncyclic hydrocarbon, the molecular formula will have the form CnHn+2

For example:

C2H6

C8H18

C20H42

Alkenes

Unsaturated hydrocarbons that contain one or more double covalent bonds

Simplest alkene has two carbons joined by a double bond

Ethene

CH2=CH2

Alkenes with More Carbons

Propene

CH2=CH-CH3

Alkenes with More Carbons

Butene

CH2=CH-CH2-CH3 & CH3-CH=CH-CH3

Alkenes with More CarbonsPentene

Alkene with SubstituentsSame basic rules as for alkanes

Alkene with Substituents5, 5 - diethyl - 2, 6, 7- trimethyl - 3 - octene

Alkene with more than one double bond

1, 3 - butadiene

Alkene with more than one double bond6-methyl-1,4,8-nonatriene

Alkynes

Unsaturated hydrocarbons that contain one or more triple covalent bonds

Same naming conventions as alkanes and alkenes

Alkynes with More Than 2 Carbons

Propyne

CH≡C-CH3

Alkynes with More Than 2 Carbons

Use initial digit to indicate where triple bond is

e.g. 1-hexyne, CH≡C-CH2-CH2-CH2-CH3

2-hexyneCH3-C≡C-CH2-CH2 -CH3

3-hexyneCH3-CH2-C≡C-CH2-CH3

Alkynes with Substituents

7-methyl-3-octyne

CH3-CH-CH2-CH2-C≡C-CH2-CH3

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CH3

VSEPR Geometry - Unsaturated Hydrocarbons

Carbon with 1 double bond and two single bonds: three groups/regions of electrons around it

Trigonal planar geometry

VSEPR Geometry - Unsaturated Hydrocarbons

Carbon with 2 double bonds: two groups/regions of electrons around it

Linear geometry

VSEPR Geometry - Unsaturated Hydrocarbons

Carbon with 1 double bonds and 1 single bond: two groups/regions of electrons around it

Linear geometry