Topic 17 organic chemistry

Question 1

What is optical isomerism?

Answer 1

A form of stereoisomerism, same structural formula but different arrangement of atoms in space.

Optical isomers are mirror images of eachother and have a chiral carbon atom

Question 2

What is a Chiral molecule?

Answer 2

A chiral molecule has 4 different groups attached to a carbon atom. We can arrange these groups in 2 different ways which forms 2 different molecules

These are called enantiomers

Question 3

What are enantiomers?

Answer 3

They are mirror images of eachother and are non-superimposable. This means no matter which way you turn them, they won’t overlap

Question 4

How do you find the chiral centre?

Answer 4

First we need to find the carbon atom with 4 different groups surrounding it. Then we need to draw it using tetrahedral 3D shape.

Question 5

How do we detect an optically active compound?

Answer 5

Optically active isomers will rotate plane polarised light. This is a method of detecting an optically active compound

Standard light will oscillate in all directions. We then pass the light through a polaroid filter to produce plain polarised light. This light only oscillates in one direction. Optically active compounds will rotate plane polarised light. 1 enantiomer rotates light clockwise, the other will rotate it anticlockwise.

Question 6

What are racemates?

Answer 6

When we have an equal amount of each enantiomer we have a racemic mixture

Racemates don’t rotate plane polarised light. The 2 enantiomers rotate light in opposite directions and they cancel out

A racemic mixture of a chiral product is often made by reacting achiral substances. When the molecules react, there is an equal chance of forming each enantiomer.

It is very difficult to adapt a reaction to only produce 1 enantiomer and it can be expensive

Question 7

Optical activity and reaction mechanisms (SN1)

Answer 7

Molecules with planar profiles can make racemic products. For example, SN1 mechanisms.
These reactions occur where we have an attack on the carbocation of a compound where a group breaks off

The reaction between Y- and the intermediate involves attacking the planar molecule, particularly the C+. As this is planar, the Y- can attack from above or below forming 2 different enantiomers.

The type of enantiomer formed will depend if the Y- ion attacks from above or the bottom.
Due to the planar nature of the carbocation, there is an even chance of the nucleophile attacking from above or below. This means we are likely to get a 50/50 mixture of both enantiomers and hence we produce a racemic mixture of products. As with all racemic mixtures, they don’t rotate plane polarised light.

Question 8

Optical activity and reaction mechanisms (SN2)

Answer 8

Reaction between a primary halogenoalkane with a Y- ion via a SN2 mechanism. With a SN2 reaction, we have a single enantiomer producing 1 enantiomer unlike the 2 produced under SN1.

In an SN2 mechanism, the Y- and CH3CH2X are reacting in 1 step and the nucleophile always attacks the opposite side of the leaving group (halogen). This means we only produce 1 product and it will rotate plane polarised light differently to the reactant.

Question 9

What are the similarities and differences of aldehydes and ketones?

Answer 9

They both have the carbonyl functional group of C=O. The difference between them is the position of the carbonyl group

Aldehydes have the C=O on an end carbon. All aldehydes have the ending -al
Ketones have the carbonyl group on an inner carbon. All ketones have the ending -one.

Question 10

Aldehyde and Ketone BP

Answer 10

They have relatively low BP in comparison to alcohols, this is all down to IMF.

Aldehydes and ketones interact with each other via London forces and permanent dipole-dipole bonds as there is no polarity within the molecules. However, unlike alcohols, they don’t have an O-H group so do not interact via the strongest IMF hydrogen bonding. This means they have a generally lower BP than their alcohol counterparts

Question 11

Aldehyde and ketone solubility

Answer 11

Tey can hydrogen bond with water and so some have the ability to dissolve in water.
The lone pair of electrons on the oxygen on the C=O can form H bonds with the hydrogen atoms on water molecules

However, only smaller aldehydes and ketones will dissolve. Larger ones have longer hydrocarbon components and are non-polar and these can disrupt the H bonding with water molecules

If the aldehyde or ketone is large enough then the London forces between the non-polar hydrocarbon chain will be stronger than the hydrogen bonding between the C=O and H on the water molecules. This means larger aldehydes and ketones won’t dissolve

Question 12

Testing for aldehydes and ketones

Answer 12

Acidified potassium dichromate oxidises aldehydes but not ketones

Aldehydes:
These are oxidised by K2Cr2O7. This is a mild oxidising agent so is reduced itself. It will turn from orange, Cr2O72- dichromate ion, to green, Cr3+ chromium ion
Cr2O72- + 14H+ + 6e- —> 2Cr3+ + 7H2O

Ketones:
Can’t be oxidised so no colour change
Tollens reagent can be made and then used to test for aldehyded and ketones

1) Add aldehyde/ketone to Tollens reagent and place in hot water bath. We don’t use a bunsen flame as aldehydes and ketones are flammable. Tollens contains [Ag(Nh3)2]+ and is added warm
Aldehydes - Tollens reduced to silver which coats the inside of the flask
Ketones - No silver precipitate is formed

[Ag(Nh3)2]+ + e- —–> Ag(s) + 2Nh3(aq)

1) Fehlings and Benedict solutions are oxidising agents and so oxidise aldehydes but not ketones. It is blue and contains Cu2+ ions.
Aldehydes - fehlings solution goes from blue solution to brick red precipitate (Cu2O)
Ketones - Remains blue

Cu2+ + e- —> Cu+

Question 13

Reduction of aldehydes and ketones

Answer 13

They can be reduced to form primary and secondary alcohols
Reducing agents such as NaBh4 dissolved in methanol and water can reduce aldehydes and ketones.

Primary alcohol —–> aldehyde
Secondary alcohol —–> ketone

Question 14

Potassium Cyanide and carbonyl group

Answer 14

Potassium cyanide reacts with carbonyl groups to produce hydroxynitriles.
This reaction occurs via a nucleophilic addition mechanism so this means a nucleophile (CN-) attacks the carbonyl group (C=O) and adds on to make a hydroxynitrile (molecule containing OH and CN group)

The + charged ion is attacked by the cyanide ion. The lone pair of electrons are donated from the CN- ion.
Immediately 2 electrons in the double bond transfer to the oxygen.
If we use an unsymmetrical ketone or aldehyde, a mixture of enantiomers is produced as seen in the optical isomerism topic

Question 15

Risks of potassium cyanide (CN) and how to reduce them

Answer 15

It is an irritant and very dangerous if ingested or inhaled
When reacted with moisture it can form the toxic gas, hydrogen cyanide

To reduce the risks:
1) Wear a lab coat to prevent clothing contamination
2) Use a fume cupboard to prevent exposure to toxic fumes
3) Wear safety goggles
4) Wear gloves while handling

Question 16

Testing for carbonyl groups

Answer 16

Bradys reagent or 2,4-dinitrophenylhydrazine (2,4-DNPH) can be used to identify a carbonyl group
Bradys reagent is dissolved in concentrated H2SO4 and methanol and then added to the substance under test. If a carbonyl group exists, a bright orange precipitate is formed. It only reacts with C=O in aldehydes and ketones not carboxylic acids

The orange precipitate is a derivative of a carbonyl compound which is purified through recrystallisation. Different carbonyl compounds produce different derivatives. They have different MP’s so they can be identified against a library of known MP

Question 17

Carbonyl reaction with iodine

Answer 17

Carbonyl groups that have a methyl group attached react with iodine.
If we heat iodine in the presence of an alkali with a methyl carbonyl group, then we will produce a yellow precipitate called trichloromethane (CHI3)
A methyl group only exists in ethanal and in ketones with a methyl group

RCOCH3 + 3I2 + 4OH- —-> RCOO- + CHI3 + 3I- + 3H2O

Question 18

Carboxylic acids

Answer 18

They have the carboxyl functional group which contains both a carbonyl group and hydroxyl group
When naming carboxylic acids, the carbon on the carboxyl group is always carbon 1. Will always be at the end of the molecule.

Question 19

Carboxylic acid solubility

Answer 19

It can hydrogen bond with water and so some have the ability to dissolve in water
The lone pair of electrons on the oxygen of the c=o can form hydrogen bonds with the hydrogen atoms on water molecules. In addition, oxygen on water molecules can hydrogen bond with hydrogen on the O-H group in a carboxylic acid.

However, just like aldehydes and ketones, only smaller carboxylic acids will dissolve. Larger ones have larger hyfrocarbon components which are non polar and these can disrupt the hydrogen bonding with water molecules

Question 20

What are dimers?

Answer 20

These can form when we have a pure, liquid for of a carboxylic acid. A dimer is where the carboxylic acid hydrogen bonds with each other. The net effect is the BP increases as the “joined” molecules are larger so the IMF is greater

Question 21

Making carboxylic acids

Answer 21

Carboxylic acids can be made from nitriles, aldehydes and primary alcohols.

They can be made from the hydrolysis of nitriles

Question 22

Carboxylic acid reactions

Answer 22

They are weak acids and react with carbonates to form CO2 and bases to form salts.
They are weak acids which means they dissociate partially to form a H+ ion and a carboxylate ion. Equilibrium lies to the left as it dissociates poorly

As they are acids, they react with Carbonates (Co3)2- to form a salt, CO2 and water