Isomerism and Carbonyl Compounds

Question 1

Describe optical isomerism:

Answer 1

Optical isomerism is a type of stereoisomerism.

Stereoisomers have the same structural formula, but their atoms arranged differently in space.

Question 2

Describe what a chiral centre is:

Answer 2

A chiral centre is, a carbon atom that has four different groups attached to it.

Question 3

Describe how a chiral centre links to optical isomerism:

Answer 3

It’s possible to arrange the groups in 2 different ways around carbon atoms so that 2 different molecules are made up - these molecules called optical isomers/enactiomers. They are mirror images and no matter which way you turn them, they can’t be superimposed.

Question 4

How do optical isomers rotate plane-polarised light?

Answer 4

Normal light vibrates in all directions.
Plane-polarised light only vibrates in one direction.
Optical isomers are optically active - they rotate plane-polarised light.
One enantiomer rotates it a clockwise direction, and the other rotates it in and anticlockwise direction.

Question 5

Define enantiomer:

Answer 5

Isomers formed in a molecule with a chiral carbon atom. They rotate plane-polarised light.

Question 6

Define racemate:

Answer 6

Racemate (or racemic mixture) contains equal quantities of each enantiomer of an optically active compound.

Question 7

What don’t racemates show?

Answer 7

Optical activity

Question 8

Why don’t racemates show optical isomerism?

Answer 8

Because the two enantiomer cancel each other’s light-rotating effect.

Question 9

Explain how reactions involving planar bonds often produce racemates. Use propanal (C3H6O) with acidified potassium cyanide (KCN) as your example.

Answer 9

1) the reaction of C3H6O + KCN involves a CN1 ion attaching the + charge on the carbon of the propanal’s planar C=O group.
2) The C- ions can attack from two directions-from above the plane of the molecule, or from below it.
3) Depending on which direction the nucleophilic attack happens from, one or two enantiomers is formed.
4) Because the C=O bond is planar, there is an equal chance that the nucleophile will attack from either of these directions.
5) So when propanal reacts with acidified KCN, you get a racemates mixture of products.

Question 10

What can be oxidised, an aldehyde or a ketone?

Answer 10

An aldehyde can be oxidised to produce a carboxylic acid but ketones can’t.

Question 11

How do you reduce an aldehyde or ketone back to an alcohol?

Answer 11

NabH4 dissolved in water with methanol is the reducing agent used.
1) Reducing an aldehyde to a primary alcohol.
R-C=O + 2[H] —> R-CH2-OH
H

2) Reducing a ketone to a secondary alcohol.
R-C=O + 2[H] —> H
H R-C-OH
R’

Question 12

Draw the nucleophilic addition reaction from when potassium cyanide is reacting with the Carbonyl.

Answer 12

See book.

Question 13

Why is potassium cyanide a dangerous chemical to work with?

Answer 13

It’s an irritant
Extremely dangerous when inhaled/eaten
Reacts with moisture to produce hydrogen cyanide, a highly toxic gas

Question 14

What is the functional group of a carboxylic acid?

Answer 14

-COOH

Question 15

How well do carboxylic acid’s dissociate?

Answer 15

Carboxylic acids are weak acids - in water they partially dissociates into carboxylate ion and H+ ion.

Question 16

What forms when the carboxylic acid reacts with carbonate?

Answer 16

Carboxylic acids react with carbonates (which contain the CO3 ^2- ion) to form a salt, carbon dioxide and water.

Question 17

Describe how an ester is formed from the reaction of an alcohol and a carboxylic acid.

Answer 17

Esters are organic compounds that contain a -COO-
They’re frequently made by heating the carboxylic acid with alcohol in the presence of a strong acid catalyst, usually concentrated sulphuric acid.

It’s also a condensation reaction as it releases water.

Question 18

What is the functional group of ester?

Answer 18

-COO-

Question 19

What are esters used as?

Answer 19

Esters are used as:
Food flavourings
Perfumes
Solvents
Plasticises

Question 20

What is hydrolysed to form an ester?

Answer 20

Alcohols

Question 21

How are esters hydrolysed to form alcohol?

Answer 21

Acid hydrolysis:
Splits the ester into an acid and an alcohol - you reflux the ester with dilute acid, such as HCl or H2SO4

Base hydrolysis:
Involves refluxing the ester with a dilute alkali, such as NaOH.
You get a carboxylate ion and an alcohol.

Question 22

What are fats and oils esters of?

Answer 22

Facts and oils are esters of glycerol and fatty acid’s.

Question 23

What is a fatty acid?

Answer 23

A fatty acid consists of a carbon chain the carboxylic acid group at one end. These a part fat and oil molecules.

Question 24

What is an unsaturated fatty acid and give an example of one.

Answer 24

And unsaturated fatty acid has double bonds, one example is vegetable oils.

Question 25

What is a saturated fatty acid and give an example of one.

Answer 25

A saturated fatty acid is a fatty acid that has no double bonds. One example is animal fats.

Question 26

What is biodiesel made up of?

Answer 26

Biodiesel is made up of a mixture of methyl esters of fatty acids.

Vegetable oils e.g. rapeseed oil, make a good vegetable fuels, but you can’t directly burn them in engines.
The oils must first be converted into biodiesel. This involves reacting with methanol, using potassium hydroxide as a catalyst.
You get a mixture of methyl esters of fatty acids-this is biodiesel.

Question 27

What method do you use to remove water soluble impurities from a product?

Answer 27

Separation.

Question 28

How does separation work?

Answer 28

1) Once the reaction to form the product is complete, pour the mixture into a separating funnel, and add water.
2) Shake the funnel and then allow it to settle. The organic layer and the aqueous layer (which contains any water soluble impurities) are immiscible, so separate out into two distinct layers.
3) you can then open the tap and run each layer off into separate container.

Question 29

How does solvent extraction work?

Answer 29

If your product and impurities are both sorry but in water, you use solvent extraction.

You take an organic solvent in which the product is more soluble in that it is in water.
You add it to the impure product solution and shake well.
The product will dissolve into the organic solvent, leaving the impurities dissolved in the water.
Use a separating funnel to get your pure product.

Question 30

How do you remove water from your purified product?

Answer 30

Add an anhydrous salt such as (MgSO4) or (CaCl2). The salt is used as a drying agent-it binds to any water present to become hydrated.

When you first add the salt to the organic layer it will clamp together. Continue adding drying agent until it disperses evenly when the flask is swirled.

Finally, filter the mixture to remove the solid drying agent-put a piece of filter paper into a final that feeds into the flask and pour the mixture into the filterpaper.

Question 31

How do you remove impurities by washing?

Answer 31

The product of the reaction can be contaminated with leftover reagent or unwonted side products. You can remove some of these by washing the project (which means adding another liquid and shaking)

One example is a create sodium hydrogen carbonate can be added to an Imp your product in solution to remove acid from it. The acid reacts with the sodium hydrogen carbonate to give CO2 gas, and the organic products can be removed using a separating funnel.

Question 32

How are volatile liquid purified by distillation?

Answer 32

1) Connect a condenser to a round bottom flask containing your impure products in solution.
2) Place a thermometer in the neck of the flask. The temperature on the thermometer will show the boiling point of the substance of operating at any given time.
3) Heat the impure product.
4) When the product that you want to collect boils, place a flask at the open end of the condenser to collect your pure product.

Question 33

Organic solids can be purified by recrystallisation, state how:

Answer 33

1) And very hot solvent to the Imp your solid until it just dissolved.
2) Filter the hot solution through a heated funnel to remove any insoluble impurities.
3) Leave the solution to cool down slowly. Crystals of the product will form as it cools.
4) Remove the liquid containing the soluble impurities from the crystals by filtering mixture under reduced pressure.
5) Finally, wash the crystals with ice cold solvent to remove any impurities from their surface.
6) Leave your purified crystals to dry.

Question 34

Why are melting and boiling points good indicators of purity?

Answer 34

Pure substances have a specific melting and boiling point.
If they are impure, the melting point’s lowered and boiling point is raised.
If they are very impure, melting and boiling will occur across a wide range of temperatures.