4.1.2 Alkanes

Are alkanes saturated or unsaturated?

saturated

What bonds are present in alkanes?

• C-C

• C-H

What type of bond is a C-C bond?

sigma bond

What type of bond is a C-H bond?

sigma bond

How is a sigma bond formed?

direct single overlap of orbitals directly between the nuclei of bonding atoms

As a sigma bond is a single overlap, what does this allow?

rotation of the bond

Describe sigma bonds

• saturated

• single

• strong

Draw a diagram of sigma bonding

What is the shape and bond angle around each carbon atoms in alkanes and why?

• 109.5

• tetrahedral

• 4 bonding pairs of electrons repel equally

What is needed to boil an alkane?

break the induced dipole-dipole attraction forces between molecules

What do you break when boiling an alkane?

induced dipole-dipole attraction forces between

Give 2 factors that determine boiling point of an alkane

• carbon chain length

• branching

As carbon chain length increases what happens to boiling point of the alkane?

boiling point increases

Why does increasing carbon chain length increase boiling point of alkane?

• more surface contact between molecules

• more induced dipole-dipole interactions between the molecules

• which require more energy to overcome

What does more surface contact mean?

more induced dipole-dipole forces between the molecules

Describe the boiling point of a branched isomer and a nonbranched isomer

a branched isomer has a lower boiling point that an unbranched isomer

Why does a branched isomer have a lower boiling point than a non-branched isomer?

• carbon chain length decreases

• less surface contact between molecules

• less induced dipole-dipole forces between molecules

• requires less energy to overcome

What intermolecular forces exist between alkane molecules?

induced dipole-dipole forces

Are C-H sigma bonds polar?

no

What type of bond breaking occurs between C-C bonds and C-H bonds?

homolytic fission

Describe the bond enpalthy of alkanes

they have high bond enpalthy

Give 2 important chemical reactions that alkanes are involved in

• combustion

• halogenation

Is combustion endo or exothermic?

exothermic because heat is being released to the surroundings

What are alkanes useful as?

fuels

How does complete combustion occur?

with a plentiful supply of oxygen

How does incomplete combustion occur?

a limited supply of oxygen

Why is incomplete combustion dangerous?

it releases carbon monoxide as a product

Why is CO toxic?

it binds to haemoglobin in the blood, preventing RBC from binding with oxygen which can be fatal

Write a balanced equation for the complete combustion of butane gas

C4H10 + 6.5O2 → 4CO2 + 5H2O

Write a balanced equation for the uncomplete combustion of octane gas

C8H18 + 8.5O2 → 8CO + 9H2O

What are the reagents for radical substitution?

X2 (halogen) and excess alkane

What are the conditions for radical substitution?

uv radiation

What is radical substitution?

a substitution type reaction because H atom in the alkane molecule is replaced by a halogen atom

Which bonds break first in radical substitution?

covalent X-X bonds within the halogen

How do the X-X halogen bonds break?

by homolytic fission

What does the breaking of X-X halogen molecule form?

two X radicals

What is a radical?

a species with an unpaired electron

How do you show radicals?

a dot is used to represent the unpaired electron

What are the limitations of radical substitution?

• structural isomers created

• mixture of haloalkanes produced

• unwanted alkane is produced from the reaction of 2 alkyl radicals

Why does radical substitution create structural isomers?

there may be substitution reactions at different positions along the carbon chain

Why does radical substitution create a mixture of haloalkanes?

further substitution could take place until all of the hydrogen atoms have been replaced

Give an example of 3 different haloalkanes that could be produced in the radical substitution of chlorine and butane

• chloromathane

• dichloromethane

• trichloromethane

Why is an unwanted alkane sometimes formed in the termination stage?

two alkyl radicals bond

How is further substitution in radical substitution avoided?

use of excess alkane

What are the 3 steps of radical substitution?

• initiation

• propogation

• termination

What is initiation of radical substitution?

the breaking of the X-X bond by homolytic fission to make two X radicals

What is the equation for initiation of radical substitution?

X2 → 2X^.

what provides the energy to break the X-X halogen bond in initiation?

uv radiation

What does the X radical do in propogation (i)?

attacks the alkane molecule

What happens in propogation?

a very reactive X^. radical attacks an alkane molecule in a chain reaction

What is the equation for propogation i

X. + alkane → alkyl radical + HX

what is the equation for propogation ii?

alkyl radical + X2 → X. + haloalkane

Why is propogation a chain reaction?

the radical is used up in the first propogation step but it is regenerated in the second, so is able to attack a new alkane molecule - continues

What is termination in radical substitution?

when two radicals combine forming a new molecule

What are the 3 possible termination steps?

• 2X^. → X2

• X^. + alkyl radical → haloalkane

• 2 alkyl radicals → alkane with 2x as many carbons as beginning alkane