Topic 3.4 - Alkenes

What is an alkene?

Unsaturated hydrocarbons with a C=C

double bond

What is the general formula

of an alkene?

C n H 2n

Why is there no rotation

about the C=C double

bond?

Due to the π (pi) orbital - electron density above

and below the single bond, which holds the

carbon atoms in place

Are they more or less

reactive than alkanes?

Why?

More reactive, due to high electron density of

double bond and the fact the pi-bond is slightly

easier to break

What intermolecular forces

of attraction do they have?

Only van der Waals due to non-polar bonds

Are they soluble in water?

Why?

No, non-polar bonds (van der Waals’ < hydrogen

bonding)

Name and describe the

three kinds of isomers

alkenes can have

Chain isomers (branched chains)

Position isomers (C=C on different carbon atom)

Geometric E-Z isomers (Z is when 2 highest atomic number

chains are on the same side of the double bond; E is when

they're on opposite sides)

Write an equation for the

complete combustion of

pent-2-ene.

CH 3

CH=CHCH 2

CH 3 + 7½O 2 → 5CO 2 + 5H 2

O

What is an electrophile?

Electron deficient atoms/ions which accept a pair

of electrons (here, from the C=C double bond)

What is the most stable type

of carbocation intermediate?

why?

Alkyl groups have a positive inductive effect, so the most

stable carbonation is the one bonded to the most other

carbon atoms i.e. A tertiary carbocation

Major products will be

formed from which kinds of

carbocations?

Tertiary (or the most stable avaliable)

What conditions are needed

for the electrophilic addition

of H 2 O to an alkene?

Acid catalyst, usually phosphoric acid

What are the product(s) of

the reaction?

An alcohol

Draw a mechanism for the

addition of water to ethene

What conditions are needed

for the electrophilic addition

of a hydrogen halide to an

alkene?

Room temperature

Draw a mechanism for the

reaction of HBr and ethene.

What conditions are needed

for the electrophilic addition

of a halogen molecule to an

alkene?

Room temperature and organic solvent

How does a molecule with a

non-polar bond react as if it

is an electrophile?

C=C double bond with a high electron density

induces a temporary dipole in the halogen

molecule → δ+ atom attracted to double bond

Draw a mechanism for the

reaction between bromine

and ethene

Draw a mechanism for the

reaction of sulfuric acid with

ethene.

How would you turn the product (from

sulfuric acid + ethene) into an alcohol

and how does this show that sulfuric acid

catalyses the addition of water to an

alkene?

Add water

H₂SO₄ reforms, showing it catalyses the

hydration of alkenes

What is an addition

polymer?

many monomers bonded together via

rearrangement of bonds without the loss of any

atom or molecule

What are monomers? What

form do they usually take?

Molecules which combine to form a polymer

Usually have a C=C bond which breaks to leave

a repeating pattern

Draw how you would

represent the polymerisation

of ethene.

Give 3 uses of

poly(chloroethene) / PVC

Drainpipes

Vinyl

Aprons

Give two examples of

plasticisers

esters and phthalates

What are plasticisers?

Small molecules that get between polymer

chains to force them apart and allow them to

slide over one another

How do the physical properties

of PVC change due to a

plasticiser? What applications

does this lead to?

PVC with a plasticiser become flexible, used for

aprons

Without a plasticiser, PVC is rigid, used for

drainpipes

Why do things containing

mainly C-C and C-H bonds

not decompose easily?

Bonds are non-polar so are not attacked by

enzymes

Why is a lack of

biodegradability in

compounds with C-C or C-H

bonds a problem?

Disposal is very problematic

What is mechanical

recycling?

Where plastics are separated into different types,

washed, ground down, melted and re-moulded

What is mechanical

recycling used for?

Soft drinks bottles → fleeces

What is feedstock

recycling?

Plastics heated to a temperature which break

polymer bonds, leaving original monomers that

can be made into new plastics

What is feedstock recycling

used for?

Making totally new plastics

What is a problem with

recycling?

Each time thermosoftening plastics are melted

and remoulded, their properties degrade, so they

can only be remoulded a limited number of times