Chapter 11:Alkanes

K. Dunlap Chem 104

Hydrocarbon:Hydrocarbon: a compound composed of only carbon and hydrogen.

Alkanes:Alkanes: hydrocarbons containing only carbon-carbon single bonds.– The first two alkanes are methane and ethane.

line-angle formula or skeletal formula:line-angle formula or skeletal formula: – A line represents a carbon-carbon bond and a line

terminus represent a carbon atom.– Hydrogen atoms are not shown in line-angle

formulas.

The first 10 alkanes with unbranched chains

Constitutional isomers:Constitutional isomers: compounds that have the same molecular formula but different structural formulas (different connectivity).– For the molecular formulas CH4, C2H6, and C3H8,

only one structural formula is possible; there are no constitutional isomers for these molecular formulas.

– For the molecular formula C4H10, two constitutional isomers are possible.

Question: Question: do the structural formulas in each set represent the same compound or constitutional isomers?

Constitutional isomersConstitutional isomers

Solution for (a):Solution for (a):

(a) They represent the same compound.

Solution for (b):Solution for (b):

(b) They represent constitutional isomers.

Problem:Problem: draw structural formulas for the five constitutional isomers of molecular formula C6H14

Solution:Solution:

Constitutional isomersConstitutional isomers

Number of Possible Isomers for Selected Alkanes

Number of Possible Isomers for Selected Alkanes

• The IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts: (1) a prefix: the number of carbon atoms in the

chain.

(2) the suffix -ane-ane: shows that the compound is a saturated hydrocarbon.

Naming Alkanes

• The name of an alkane with a branched chain of carbon atom consists of:– a parent name: the longest chain of carbon atoms– substituent names: the groups bonded to the

parent chain

Naming Alkanes

a substituent group derived from an alkane by removal of a hydrogen atom.– commonly represented by the symbol R-.– named by dropping the -aneane from the name of the

parent alkane and adding the suffix -ylyl.

Alkyl groups:

1. The name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms and the ending -ane.ane.

2. For branched-chain alkanes, the longest chain of carbon atoms is the parent chain and its name is the root name.

3. Name and number each substituent on the parent chain; use a hyphen to connect the number to the name.

Naming Alkanes

4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number.

Naming Alkanes…

5. If the same substituent occurs more than once:– Number the parent chain from the end that gives

the lower number to the substituent encountered first.

– Indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa-, and so on.

– Use a comma to separate position numbers.

Naming Alkanes…

6. If there are two or more different substituents:– list them in alphabetical order.– number the chain from the end that gives the lower

number to the substituent encountered first.– If there are different substituents in equivalent

positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number.

Naming Alkanes…

7. Do not include the prefixes di-, tri-, tetra-, and so on;

alphabetize the names of substituents first, and then insert these prefixes

Naming Alkanes…

• Cyclic hydrocarbon: Cyclic hydrocarbon: a hydrocarbon that contains carbon atoms joined to form a ring.

• Cycloalkane:Cycloalkane: a cyclic hydrocarbon in which all carbons of the ring are saturated. – Cycloalkanes of ring sizes ranging from 3 to over

30 carbon atoms are found in nature.– Five-membered (cyclopentane) and six-membered

(cyclohexane) rings are especially abundant in nature.

Naming Cycloalkanes:

• Nomenclature– To name a cycloalkane, prefix the name of the

corresponding open-chain alkane with cyclo-,cyclo-, and name each substituent on the ring.

– If there is only one substituent on the ring, there is no need to give it a location number.

– If there are two substituents, number the ring beginning with the substituent of lower alphabetical order.

Hydrocarbon Rings

Hydrocarbon Rings

Name these alkanes

• any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond.– following are three conformations for a butane

molecule.

Conformation of alkanes

• The most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity.– The electronegativity difference between carbon

and hydrogen is 2.5 - 2.1 = 0.4 on the Pauling scale.

– Given this small difference, we classify a C-H bond as nonpolar covalent.

– Alkanes are nonpolar compounds and the only interaction between their molecules are the very weak London dispersion forces.

Physical properties of Alkane

Melting and boiling points of Alkanes:– Boiling points of alkanes are lower than those of

almost any other type of compound of the same molecular weight.

– In general, both boiling and melting points of alkanes increase with increasing molecular weight.

• Solubility: a case of “like dissolves like”.– Alkanes are not soluble in water; they are unable

to form hydrogen bonds with water.– Alkanes are soluble in each other.– Alkanes are also soluble in other nonpolar organic

compounds, such as toluene and diethyl ether.

• Density– The average density of the liquid alkanes listed in

Table 11.4 is about 0.7 g/mL; that of higher-molecular-weight alkanes is about 0.8 g/mL.

– All liquid and solid alkanes are less dense than water (1.0 g/mL) and, because they are insoluble in water, they float on water.

Physical properties Cont…

1) Natural gas– 90 to 95 percent methane – 5 to 10 percent ethane, and – a mixture of other relatively low-boiling alkanes,

chiefly propane, butane, and 2-methylpropane.

2) Petroleum– A thick, viscous liquid mixture of thousands of

compounds, most of them hydrocarbons formed from the decomposition of marine plants and animals.

Where do hydrocarbons come from?

Petroleum Distillation

Some Common Hydrocarbons:

-fuels

-paraffin

-petrolatum

-plastics

-natural plant waxes

More Naming…….