Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons

Question 2

State what is meant by the term electrophile

Answer 2

An electron pair acceptor

Question 3

Name the reaction that occurs when an alkene is turned into an alkane (eg C2H4 + HBr – > C2H5Br)

Answer 3

Electrophilic addition

Question 4

Describe and explain the electrophilic addition of HBr to C2H4

Answer 4

The HBr molecule = polar (Br = more electronegative) --> H = electrophile
C=C makes this area v. negative
Opposites attract - electrophile attracted to double bond
However, Br slightly -ve, so is repelled and breaks off - takes the electrons
The electrophile (H) accepts electrons from a C and bonds --> other C now +ve (carbocation)
Carbocation attracts -ve Br - bonds

Question 5

Why are major and minor products formed?

Answer 5

Some carbocations are more stable and so are more likely to be formed, more likely to hang around and bond with Nu (and vice versa)

Question 6

What is a tertiary carbocation?

Answer 6

A C with 3 C groups around

Question 7

State the stability of carbocations from least stable to most stable

Answer 7

methyl, primary, secondary, tertiary

Question 8

What is the positive inductive effect?

Answer 8

Alkyl groups push electrons away from themselves towards C+

Question 9

Why can electrophilic addition by Br2 occur?

Answer 9

High electron density repels the electrons in the Br2 as it nears, inducing a dipole

Question 10

Electrophilic addition of what allows sulfuric acid to be reformed? What is the other product?

Answer 10

H2O (so hydrolysis reaction) to alkyl hydrogen sulfates

An alcohol is also produced

Question 11

State what is meant by a polymer

Answer 11

Long chain molecule made from lots of small molecules joined together

Question 12

State what is meant by a monomer

Answer 12

Small molecules that join together to make polymers

Question 13

State what is meant by addition polymerisation

Answer 13

Formation of long chain molecules from lots of small molecules together with no other products

Question 14

What are the conditions for addition polymerisation?

Answer 14

Pressure, high temperature, catalyst

Question 15

Describe and explain addition polymerisation

Answer 15

Addition of something causes the double bond to break
1C is happy, but now 1 is missing a bond, so ‘steals’ one from the next molecule (causing that double bond to break)
Though this C is now happy, the 1C is not and is missing a bond and so ‘steals’ one from the next molecule, bonding with it…and so on

Question 16

Plastics are not biodegradable. This means what?

Answer 16

They cannot be broken down by enzymes, acids or alkalis

Question 17

Why are plastics non-biodegradable?

Answer 17

To break down poly(alkanes) you have to break a non-polar C-C bond. Most bonds are broken down because they are polar or by hydrolysis (polar molecule itself)

Question 18

What is a plasticiser?

Answer 18

A small molecule that gets between the polymer chains, allowing them to slide past each other. This makes them more flexible

Question 19

State the meaning of the term stereoisomers

Answer 19

Same molecular and structural formula but different spatial arrangement

Question 20

A chemical test can be used to distinguish between saturated and unsaturated compounds. State the reagent, and what you would observe with each

Answer 20

Bromine
Saturated - stays orange
Unsaturated - Turns colourless

Question 21

The formation of ethanol from ethene uses conc. H2SO4 and proceeds in 2 stages.
S1 C2H4 + H2SO4 –> C2H5OsO2OH
S2 C2H5OSO2OH + H2O –> H2SO4 + C2H5OH
State the overall role of sulfuric acid in this process

Answer 21

It acts as a catalyst

Question 22

What are some typical uses of PVC poly(chloroethene)?

Answer 22

Depending on the type of PVC, can be used in underground pipes, or packaging