Alkanes and Alkenes

What are the conditions needed for electrophilic addition with hydrogen gas?

Nickel catalyst and 150 degrees celsius

What are the conditions needed for electrophilic addition with bromine and hydrogen halide?

No catalyst required, room temperature

What are the conditions needed for electrophilic addition with steam?

Concentrated phosphoric acid catalyst, high temperature, and high pressure

Describe the chemical test for an alkene function group

Add bromine and shake. If bromine is decolourised from orange to colourless, the compound is an alkene. If bromine is not decolourised, the compound is not an alkene.

Why are alkenes more reactive than alkanes?

Due to the presence of the alkene functional group and the C=C double bond

Describe addition type reactions that alkenes undergo

The π-bond breaks when an alkene reacts due to the relatively low bond enthalpy of the π-bond. Only one product is formed and the atom economy is 100% as there’s no waste product.

What is an addition reaction?

A group is added across the C=C double bond

What is an electrophile?

An electron pair acceptor

Why do alkenes attract electrophiles?

The C=C double bond is a region of high electron density

What do the curly arrows show in an electrophilic addition mechanism?

The movement of an electron pair, to either break or make a covalent bond.

What is formed during addition reactions with unsymmetrical alkenes?

A mixture of two organic products

What is Markownikoff’s rule?

When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atom with the large number of hydrogen atoms (and the smaller number of carbon atoms).

How do you determine the major or minor product?

You need to look at the carbocation intermediate. The carbocation is classified by the number of alkyl groups (represented by -R) attached to the positively charged carbon atom. The tertiary carbocation is the most stable so forms the major product over the secondary and primary carbocation.

Why are tertiary carbocations the most stable?

Alkyl groups donate electrons towards the positive charge of the carbocation which allows the charge to spread over the alkyl groups. The more alkyl groups attached, the more the charge is spread out, making the ion more stable.

What are the conditions for additional polymerisation of alkenes?

High temperature and pressure and a catalyst

What is poly(ethene) used for?

Packaging and polythene bags

What are the problems with the disposal of addition polymers?

Non biodegradable, burning (incineration) produces toxic gases

Why are additional polymers non-biodegradable?

The C chain of an addition polymer is a non-polar and cannot be broken by hydrolysis. Addition polymers are therefore very stable and do not break down naturally in the environment or at landfill sites by living organisms.

What are the benefits for sustainability of processing waste polymers?

• Combustion for energy production

• Removal of toxic waste products

• Use as an organic feedstock for the production of plastics and other organic chemicalsWhat

What are stereoisomers?

Compounds with the same structural formula but with a different arrangement of the atoms in space.

What are the criteria for a compound to show E/Z isomerism?

• Must have a C=C double bond because this cannot rotate

• Each carbon of the C=C must have two different groups attached to it

How are atoms attached to each carbon atom in a double bond given priority?

Atomic number

What is a Z isomer

If the groups of higher priority are on the same side of the double bond

What is a E isomer?

If the groups of higher priority are on diagonally opposite sides of the double bond

What is the criteria for cis-trans stereoisomerism?

• Must have a C=C double bond because this cannot rotate

• Each carbon on the C=C must have two different groups attached

• Two groups on the C=C bond must be identical

What is a trans isomer?

Two same groups are opposite the C=C bond (across from each other)

What is a cis isomer?

Two same groups are adjacent the C=C bond

Why is the shape around the C=C double bond trigonal planar?

• There are three bonding regions of electron density around each of the carbon atoms

• The three bonding regions repel each other equally to get as far apart as possible, so the bond angle around each carbon atom is 120 degrees

• All of the atoms are in the same plane

Why is energy needed to boil a liquid alkane?

Energy is needed to break the induced dipole-dipole attraction forces between the molecules (in order to separate the molecules from one another)

Why does boiling point increase as chain length increases?

Because there is more surface contact between molecules (and more molecules) so there are more induced dipole dipole interactions between the molecules which need more energy to overcome.

Why does a branched isomer have a lower boiling point than an unbranched isomer?

• In a more branched alkane there is less surface contact between molecules - leading to fewer London forces

• Which need less energy to break the weaker induced dipole-dipole interactions between molecules

Why are alkanes relatively unreactive?

The C-C and C-H sigma bonds are strong and non=polar. Alkanes also have a high bond enthalpy which gives them a low reactivity.

What are alkanes useful for?

As fuels

Why is carbon monoxide dangerous?

It prevents haemoglobin in red blood cells binding with oxygen, which can be fatal

What are the conditions needed for radical substitution?

UV radiation

What is a radical?

A species with an unpaired electron

How are bonds typically broken during radical substitution?

Homolytic fission

What are the limitations of radical substitution?

• Further substitution

• Structural isomers are formed

• Low % yield of desired haloalkane so separation by fractional distillation of the desired haloalkane from the product mixture is needed which is costly

How do you minimise the probability of further substitution?

An excess of alkane is used