Module 6 Need To Know

Question 1

Old kekule model

Answer 1

• 6 cabron hexagonal ring
• with alternating pi bonds
• and therefore localised electron density in those areas

Question 2

3 pieces of evidence to why the kekule model was wrong

Answer 2

1) techniques allowed to find the length of sigma and pi bond, with pi bond being smaller than sigma. However in benzene all the bonds were of same lenght

2) it was expected kekule model could decolurise bromine water and take part in electrophilic additon reactions like alkenes. However it couldn’t

3) the enthalpy of hydrogenation for benzene was expected to be 3 times rhe value for cyclohexene, as it had 3 x more double bonds . However it had a less negative value, and thus was more stable

Question 3

New delocalised model for benzene

Exam wording, remember the last few points

Answer 3

• benzene is a 6 carbon hexaganol planar ring
• one electron is used per carbon to form sigma bonds with hydrogen atoms thst also are in the same plane
• 2 electrons are used to form sigma bonds with adjacent cabron atoms
• the 4th electron sits in a P ORBITAL 90° to the plane
• adjacent p orbitals overlap in BOTH DIRECTIONS!!! to form a continous ring of electron density above and below the ring
• this forms a SYSTEM of pi bonds, where the electrons are said to be DELOCALISED

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Question 4

In general why are delocalised molecules more stable than localised ?

Answer 4

Electron density is spread over atoms so more stable

Aromatic always stable

Question 5

Why can’t benzene undergo electrophikic additon compared to alkene ?

Last point

Answer 5

• alkene has localised electron density between 2 carbon atoms
• benzene has delocalised electron density over the ring
• as a result, any 2 carbon atoms in benzene will have less electron density compared to the alkene carbons
• thus will induce a WEAKER DIPOLE, attract a dipole LESS, polarises it less , and so resistance to brominstion and electrophikic attack is HIGHER

Z

Question 6

What are aromatic compounds used in industry
Learn

Answer 6

Used to make pharmaceuticals
- dyes
- explosives like tnt

Question 7

Why is the reduction to form hydroxynitriles , and alkylation IMPORTSNT

Answer 7

Only few reactions which can increase the carbon chain length

Question 8

Test for carbonyl group

Answer 8

Soltuion of 2-4 DNP

this produces an orange PRECIPATE if found positive ( so aldehyde and ketone )

Question 9

How to distinguish between alhdyde and ketone

What do we use
WHY, colour change, what’s produced after reaction happens

Answer 9

Use tollens reagent
- this is a solution of silver nitrate in aqueous ammonia
- the silver ions reduce to become silver, thus a SILVER MIRROR IS FORMED

Howeber when the silver ions are reduced, then the aldehyde is oxisided, and becomes a CARBOXYLIC ACID

As ketone can’t be oxidised, this test perfectly identifies ketone from aldehydes

Question 10

Why are carbox acids soluble

What happens when length of alkyl chain increases

What happens when another Carboxyl group added

Answer 10

They can for, hydorgen bonds in 2 postions with water, one fro, c=O one with OH, hence it is soluble , as energy released from hydrogen

One again, increase alkyl chain, the effect of the carbonyl group has less effect as the non polar chain dominates

Adding another = increased solubility

Question 11

What is ester
- how made ( reactants, conditions , products)

Answer 11

Ester = condensation reaction
- alchol carbox / acyl or phenol acyl
- CONC SULFRUIC ACID NEEDED
- produces ester with ester bond and water / hcl

Question 12

What is a condensation reaction

Answer 12

Addition immediately followed by elimination

• forms a big and small molecule, water or hcl

Question 13

2 types of hydrolysis for esters / amide

What is required for both

What is returned for bath !

Answer 13

1) acid hydrolysis requires HOT AQEUOS H+ (acid)
- this returns the carboxylic acid and the alchol COMPLETLEY
- reverse of esterificsrion, reflux with sukfruic acid catlsydt agsin

2) base hydrolysis requires HOT AQEOUS ALKALI
- this returns the alchol but
- the carboxylic acid becomes a SALT

Don’t forget, only applied to CONH / COOH

Question 14

Acid vs base hydrolysis , which is reversible etc

Answer 14

Acid is reversible ( it’s just the reverse process of estericstiom )

Acid one is non reversible as salt is made

Question 15

How to do acid hydrolysis in detail

Answer 15

Reflux with sukfruic acid
ADD WATER as this was lost

This breaks them down

Question 16

How to make an acyl chloride (IMPORTANT )

Where should you carry out this reaction

Answer 16

THINOYL chloride SOCL2 + parent carbox
= acyl chlroide + so2 +Hcl

2) must carry out in fume cupboard because so2 and hcl are HARMFUL

Question 17

Reactions of acyl chlorides
4 separate ones

Answer 17

1) acyl + alcohol = ester
Acyl + PHENOL = ester too (only acyl can do because reactive enough)
2) Acyl + water =carbox (like all of them)
3) acyl = ammonia / amine = primary / second AMIDES

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Question 18

Why can ONLY ACYL chlroides react with phenol to produce ester, why Can’t carbox do too?

Answer 18

Whole point in using acyl chloride is it’s MORE REACTIVE than carboxylic acids

Carbox aren’t reactive enough, must be acyl for ph roll to produce ester

Question 19

Acyl chlroide + ammonia / AMINE

Answer 19

1) with ammonia forms a PRIMARY amide , + salt
2) with primary AMINE ( not amide) forms secondary AMIDE

Reactions are kinds tek, need to balance salt use two amines etc

Question 20

Acid anhydride reactions

Answer 20

All the same as acyl chrlodie
1) alcohols and phenol to form ester
2) water to form carobykic acid
3) amine and ammonia to form amides

Question 21

So why would we use acid anhydride vs Aycl chlrodie vs carbox ?

Answer 21

Acyl and acid Anhydide more reactive than carbox like they can react with esters

• acyl chlroides gery reactive, so sometimes want less reactive they use acid anhydride