Module 4 Section 1

Question 1

Define homologous series

Answer 1

A group of organic compounds with the same functional group where each successive member differs by CH2.

Question 2

What is an aliphatic compound?

Answer 2

Contain hydrogen and carbon joined in straight change, branches chains or non-aromatic rings

Question 3

What is an alkyl group?

Answer 3

A ‘fragment’ of a molecule with general formula CnH2n+1

Question 4

What is a structural isomer?

Answer 4

Two molecules that have the same molecular formula but are arranged differently

For example, you might get a hydrocarbon with an alkyl group on the side of it

Question 5

What shape are alkane molecules around each carbon atom?

Answer 5

Tetrahedral

Question 6

What happens to the boiling point of alkanes as they get longer?

Answer 6

The boiling point gets higher

Stronger London forces between the molecules as more atoms are involved (so more electrons). There’s also more surface contact

Question 7

Why do branched molecules have lower boiling points than their straight-chained isomer?

Answer 7

When they’re branched, molecules can’t pack as closely to each other so the attractions between the molecules is much weaker.

Question 8

What is heterolytic fission?

Answer 8

Bond breaks unevenly with one of the bonded atoms receiving both electrons from the bonded pair

You get a cation (+) and an anion (-)

Question 9

What is homolytic fission

Answer 9

Bond breaks evenly, both atoms receive one electron from the bonded pair

Makes 2 radicals

Question 10

3 steps of free radical substitution?

Name and general description

Answer 10

1. Initiation - Free radicals made by UV light causing homolytic fission - ‘photodissociation’
2. Propagation - Free radicals used up and new ones made in chain reactions
3. Termination - free radicals react together to make stable compounds

Question 11

Essential condition to react halogens with alkanes?

Answer 11

UV light

Question 12

2 problems with free radical substitution reactions?

Answer 12

• you get a mixture of products. You almost always end up with a radical at the end of each reaction, so another reaction can occur after that and so on
• might form isomers of the thing you’re after, ie the halogen might be in the wrong place

Question 13

What is meant by ‘unsaturated’

Answer 13

Molecule with at least one carbon-to-carbon double covalent bond

Question 14

How is a Pi bond formed?

Answer 14

Pi bonds are formed by the sideways overlap of P orbitals above and below the 2 carbon atoms

See pg 92 of revision guide for the fancy diagrams which exam Q’s give you marks for if you draw it relevantly to a question

Question 15

What is a stronger bond, a sigma bond (2 s-orbitals overlapped) or a Pi bond?

Answer 15

A sigma bond is what a single bond has between carbon atoms. A double bond has a sigma bond and a Pi bond

There is a greater electron density with the sigma bond than the Pi bond, so the electrostatic attraction is greater. Higher bond enthalpy.

Look at pg 92 for diagrams and shit

Question 16

Why can’t atoms rotate around c=c bonds

Answer 16

Because of the way P-orbitals overlap to form a Pi bond.

They don’t bend much either (rigid)

C=C bonds are planar, so the atoms are arranged in a trigonal planar shape around them

Question 17

What is stereoisomerism?

Answer 17

Same structural formula but different arrangement in space

i.e. the atoms are just in different places within the same shaped structure

Question 18

What causes E/Z isomers in alkenes?

Answer 18

Lack of rotation around double bond can result in stereoisomers (look at page 93 for example)

Both carbon atoms in the double bond need 2 different atoms and groups on them.

These can be the same as the opposite carbon atom, but the groups on one carbon atom have to be different. (They can be the exact same as the other carbon atom though)

Question 19

How do you determine whether it’s an E or a Z isomer when the 2 carbon atoms have the same groups as each other?

Answer 19

When the same two groups are either both on the top or both on the bottom, it’s Z

When they’re diagonal to each other, it’s E

Question 20

What happens when you have completely different groups attached to an EZ isomer?

Answer 20

You look at which groups are given the ‘highest priority’, (which one on each carbon atom) and it’s E or Z depending whether they’re the same or opposite

This is decided by the atom that is DIRECTLY BONDED to the carbon, which of them has the highest atomic number

If it’s the same on both, you then look at the atoms directly bonded to that atom, and so on, till you can decide which has the highest priority

Question 21

When do cis-trans isomers occur

Answer 21

When an E/Z isomer has at least one group in common on each carbon atom

• cis is when they’re on the same side (ie both on the bottom or both on the top)
• trans is when they’re on the opposite sides (ie one on the top, one on the bottom)

Question 22

What is an electrophile?

Answer 22

They’re ‘electron pair acceptors’, they want electrons.

They’re attracted to areas where there’s weak attractions to electrons and stuff

Question 23

What happens in an electrophilic addition reaction (with alkenes)?

Answer 23

Double bond opens up, atoms are added to the carbon atoms [which had the double bond before]

Question 24

Outline the electrophilic addition mechanism

Answer 24

• heterolytic fission of reagent, electrons go to the negative side, positive bonds to one of the carbon atoms
• negatively charged part attacks the carbocation, bonds with the other carbon atom

Question 25

What do you do to test for carbon double bonds?

Answer 25

Bromine water test

An alkene will decolourise the orange bromine water when you shake them together

Question 26

How do you make an alcohol from an alkene?

Conditions?

Answer 26

Steam hydration, reacting with water in the form of steam

300 degrees C, 60-70 atmospheres and a solid phosphoric(V) acid catalyst

Question 27

What is a primary carbocation?

Answer 27

A carbocation with only one alkyl group attached to it

Question 28

What is a secondary carbocation?

Answer 28

One with two alkyl groups attached

Question 29

What is a tertiary carbocation?

Answer 29

One with three alkyl groups attached

Question 30

Which carbocation is most stable and therefore more likely to form?

Answer 30

Tertiary is most stable because the alkyl groups feed electrons towards the positive charge

Question 31

What is Markownikoff’s rule?

Answer 31

The major product is the one where hydrogen adds to the carbon with the most hydrogen’s already attached

Question 32

Problem with polymers?

How can we get rid of them?

Answer 32

They’re very unreactive, hard to get rid of

• landfill - done when separation is hard or not worthwhile. It’s not great though cause of the amount of waste we produce
• Reusing them - melting and remoulding, cracking into monomers, can be used as an ‘organic feedstock’ to make more plastics and stuff