Alkenes

What is a sigma (σ) bond

A bond with the electron cloud lying between two atoms.

What is a pi (π) bond

A bond where the electron cloud is placed above and below the two atoms (two electron clouds for one bond)

What is stereoisomerism

Molecules with the same molecular and structural formula, but a different spacial arrangement of atoms

Why is the rotation about the C=C axis restricted

There is electron density in the pi bond above and below the sigma bond which prevents rotation

When does E-Z isomerism (cis-trans isomerism) occur

When there is a double bond and two different groups on each of the carbon atoms making up the double bond.

Explain why alkenes are mire reactive than alkanes

Because of the double bond being able to break which allows each carbon to form a new bond which is often energetically favourable.

What is involved in the hydration of alkenes

Heating an alkene with steam and a phosphoric acid catalyst to form an alcohol

Give the equation of the hydration of ethene

CH2CH2 + H2O → CH3CH2OH

What is involved in the oxidation of alkenes

Adding acidified potassium manganate (VII) as a catalyst forming a diol

Give the equation for the oxidation of alkenes with acidified potassium manganate (VII)

CH2=CH2 + [O] + H2O → CH2(OH)CH2(OH)

What is involved in the hydrogenation of alkenes

Hydrogen is added to an alkene in the presence of a nickel catalyst forming a saturated alkene from an unsaturated one.

Give the equation for the hydrogenation of ethene

CH2=CH2 + H2 → CH3CH3

Where is hydrogenation useful

In the manufacturing of margarine (saturated) from oils and fats

Why do saturated fats tend to be solids at room temperature and pressure

Because they have higher melting points than unsaturated fats which tend to be liquids.

What initiated electrophilic addition in alkenes

The electron rich are in the double bond being “attacked” by a hydrophilic molecule.

What is an electrophile

An electron pair acceptor

Describe Markovnikov’s rule

Alkyl groups exhibit a positive inductive effect which pushed electron density towards carbocations. The more alkyl groups the more positive inductive effect there is and the more stable the carbocation.

What is produced when alkenes react with cold concentrated sulphuric acid

Alkyl hydrogensulphates

Why are addition polymers inert and non-biodegradable

They are non polar and the C-C and C-H bonds are very strong so require much energy to break

How do plasticisers work

They are inert substances embedded between polymer chains. They separate the polymer chains and disrupt the intermolecular forces between them. This weakens intermolecular forces between them and allows the rigid polymers to become more flexible

What is an advantage of incinerating polymers

• Energy produced can be used to generate electricity

• Volume of waste is reduced

What is a disadvantage of incinerating polymers

It releases toxic and greenhouse gases into the atmosphere

What is an advantage of recycling

Reduces use of more raw, non-renewable resources

What is a disadvantage of recycling

Requires collecting and sorting which requires energy

What is an advantage of cracking polymers

• They are more useful in shorter chains

• Converts waste polymers into useful reagents which can be used in a variety of means

What is a disadvantage of cracking

Expensive due to high energy requirement

Define biodegradable

Polymers are broken down naturally over time (micro-organisms)

What is an advantage of biodegradable polymers

Reduces amount of waste accumulation over time

What is a disadvantage of biodegradable polymers

• Microplastics/harmful substances are released into the environment

• Takes a long time to be broken down/decompose, still occupies space

What is the mechanism in which addition reactions in alkenes take place

Electrophilic substitution