C16 - Organic Synthesis

Question 1

How is an alkane formed from an alkene?

Answer 1

By hydrogenation (using H2) in the presence of a nickel catalyst at 403K.

It’s an example of electrophilic addition.

Question 2

What type of reaction is hydrogenation (forming an alkane from an alkene)?

Answer 2

Electrophilic addition.

Hydrogen atoms are electrophile.

Question 3

What is the test for alkenes?

Answer 3

Reacting it with bromine water.

If an alkene is present, the bromine water turns from orange to colourless due to the breaking of the double bond in the alkene.

Question 4

How is a haloalkane formed from an alkene?

Answer 4

By the use of a hydrogen halide.

This is an example of electrophilic addition as the hydrogen halide undergoes heterolytic fission.

Question 5

What type of reaction forms a haloalkane from an alkene?

Answer 5

Electrophilic addition due to heterolytic fission of a hydrogen halide.

Question 6

How can a haloalkane be formed from an alkane?

Answer 6

By radical substitution due to halogens gaining energy from UV radiation and undergoing homolytic fission to form radicals.

They then form a chain reaction.

Question 7

What type of reaction forms haloalkane from alkanes?

Answer 7

Radical substitution.

This is due to homolytic fission of halogens by UV radiation.

Question 8

How can an alkene form an alcohol?

Answer 8

By hydration and electrophilic addition.

This is done in the presence of H2O and H3PO3.

Question 9

What type of reaction forms alcohol from alkenes?

Answer 9

Electrophilic addition / hydration

Question 10

What is required for the electrophilic addition of an alkene to form an alcohol?

Answer 10

Water (gas/steam) and phosphoric acid for the alkene to be hydrated.

Question 11

How is an alkene formed from an alcohol?

Answer 11

The alcohol is dehydrated in an elimination reaction requiring sulphuric acid and forming an alkene and water.

Question 12

What type of reaction forms an alkene from an alcohol?

Answer 12

Elimination / dehydration in the presence of concentrated sulphuric acid.

Question 13

What is necessary for the dehydration of an alcohol?

Answer 13

Concentrated sulphuric acid.

Question 14

How can an alcohol be formed from a haloalkane?

Answer 14

By nucleophilic substitution under reflux using NaOH (aq)

Question 15

What type of reaction forms an alcohol from a haloalkane?

Answer 15

Nucleophilic substitution

Question 16

How is a haloalkane formed from an alcohol?

Answer 16

The alcohol is heated under reflux with sulphuric acid and a sodium halide, resulting in nucleophilic substitution.

Question 17

What conditions are required for a haloalkane to be formed by an alcohol?

Answer 17

Nucleophilic substitution would occur under reflux in the presence of a sodium halide and sulphuric acid.

(A hydrogen halide and sodium hydrogen sulphate are formed in situ)

Question 18

How do you identify which haloalkane is present in a sample?

Answer 18

You hydrolyse the haloalkane then add silver nitrate to find which halide is present.

White precipitate - silver chloride
Cream precipitate - silver bromide
Yellow precipitate - silver iodide

Question 19

How fast are haloalkanes hydrolysed?

Answer 19

The rate at which haloalkanes are hydrolysed / carbon-halide bonds are broken depends on bond enthalpy not electronegativity.

C-I has a low enthalpy so is hydrolysed quickly whereas C-F has a higher enthalpy so is hydrolysed slower.

Question 20

What are the two types of alcohol that can be oxidised to form new products?

Answer 20

Primary and secondary alcohols.

Question 21

What substance is used as an oxidising agent when oxidising alcohols?

Answer 21

Acidified potassium dichromate (K2Cr2O7 / H2SO4)

It turns from orange to green when reduced.

Question 22

How are aldehydes made (from alcohols)?

Answer 22

A primary alcohol is heated under reflux (in the presence of potassium dichromate) then distilled to prevent further reflux.

Question 23

How are carboxylic acids made (from alcohols)?

Answer 23

A primary alcohol is heated under continuous reflux (in the presence of potassium dichromate).

Question 24

How is a ketone formed (from an alcohol)?

Answer 24

A secondary alcohol is heated under reflux (in the presence of potassium dichromate)

Question 25

What are the quick fit apparatus?

Answer 25

Round bottom/peak shaped flask

Receiver

Screw-tap adaptor

Condenser

Still head

Question 26

How can organic compounds be heated slowly?

Answer 26

By reflux

Question 27

What equipment is needed for heating under reflux?

Answer 27

Round bottom/pear shaped glass

Condenser

Rubber tubing

Stand and clamp

Heat source

Question 28

Why are anti-bumping granules necessary when heating under reflux?

Answer 28

So that the contents boil slowly.

If not used, large bubbles will form at the bottom of the liquid and make the glassware vibrate or jump violently.

Question 29

What happen if a stopper is placed on top of the condenser during reflux?

Answer 29

Pressure builds up as the heated air expands due to the stopper resulting in a closed system. This can cause the equipment to explode.

Question 30

In what way does water flow through the condenser?

Answer 30

In through the bottom and out from the top to ensure the outer jacket is full and the tube remains as cool as possible.

Question 31

What’s distillation used for?

Answer 31

To separate a liquid from its impurities.

Question 32

What equipment is used for distillation?

Answer 32

Round bottom/pear shaped flask

Condenser

Rubber tubing

Heat source

Clamp and stand

Screw-cap adaptor

Receiver adaptor

Still head

Thermometer

Question 33

How are organic products purified?

Answer 33

When preparing organic liquids, water may also be produced, producing two layers within the collection flask.
To prevent disposing of the wrong product, water can be added and the layer which increases in size is the aqueous layer which can be removed using a separating funnel:

1) Ensure the separating funnel tap is closed.
2) Pour the mixture into the separating funnel, add a stopper and invert to mix the the contents.
3) Allow the layers to settle
4) Add water to see which gets bigger (aqueous layer)
5) Place conical flask under funnel, remove stopper and empty the lower layer of liquid.
6) Use another flask to collect second layer.

Question 34

What’s the procedure for drying an organic liquid?

Answer 34

1) Add the liquid to a conical flask
2) Add some drying agent (e.g. anhydrous calcium sulfate) and gently swirl the the contents to mix together.
3) Place a stopper on the flask to prevent evaporation of products and leave for 10 minutes.
4) If the solid has formed a lump, water is still present. Add more drying agent until the solid resembles a fine powder.
5) Decant the solid from the liquid. If dry, the liquid should be clear.

Question 35

What are the different functional groups? (10)

Answer 35

Alkene C=C
Alcohol -OH
Halo alkane -Cl -Br -I
Aldehyde -COH
Ketone =O
Carboxylic acid -COOH
Ester -COO
Amine -NH2
Acyl chloride -COCl
Nitrile -CN

Question 36

What’s a target molecule?

Answer 36

The compound the chemist is attempting to prepare by organic synthesis.
Synthesis can occur in mor than one stage.