U4 Reaction pathways

Question 1

Test for carboxylic acids?

Answer 1

Add to water and test for pH. (Weak acid so lower than 7 pH)

Question 2

Test for amines?

Answer 2

Add to water and test for pH. (Weak base so higher than 7 pH)

Question 3

Testing for alkenes or alkanes?

Answer 3

Bromine water test. Alkenes (unsaturated) will react with Br2 –> 2-Dibromoalkanes. This will be colourless. Bromine water itself is brown. Alkanes will not react and the solution will stay brown.

Question 4

Testing for primary, secondary, tertiary alcohols?

Answer 4

Add acidified potassium dichromate/potassium permanganate. Primary & Secondary will change colour from yellow to green. Tertiary will not react, wont change colour.

Question 5

What are addition reactions? List all addition reactions.

Answer 5

Addition reactions are where atoms add across a double bond in an unsaturated carbon. Bond breaks and two atoms are joined.
- Hydrogenation
- Halogenation
- Hydrohalogenation
- Hydration

Question 6

What is hydrogenation reaction + conditions?

Answer 6

Hydrogenation is where hydrogen is added to form alkane. Can also be classified as reduction as there is a gain of hydrogens. Metal catalyst is required, such as platinum or palladium.

Alkene + H2 –(Pt)–> Alkane

Question 7

What is halogenation reaction + condiitons?

Answer 7

Adding two halogen atoms to an alkene to form a dihaloalkane. No heat. No catalyst.

Alkene + Cl2 —> Dichloroalkane

Question 8

What is hydrohalogenation reaction + conditions?

Answer 8

Adding a hydrohalide (HCl) across double bond of alkene. No heat. No catalyst. Must follow Markonikov’s rule. (Hydrogen goes to carbon with most hydrogens).
The product according to the rule is the ‘Major product’, the wrong one is ‘Minor’

Alkene + HCl —> Chloroalkane

Question 9

What is hydration reaction + conditions?

Answer 9

Adding of H2O to an alkene to form an alcohol. H and OH is added on. High temperatures required (Above 100C), high pressures and a catalyst like a strong acid.

Alkene + H2O –(Acid)–> Alcohol.

Question 10

What is polymerisation reaction + conditions?

Answer 10

When two alkene compounds undergo addition polymerisation to form long polymer chains. Can also be condensation like in proteins. Requires heat +high pressures + catalyst.

Question 11

What are substitution reactions?

Answer 11

Involves replacing a substituent on a saturated compound with another substituent. Only occurs on Alkanes and Haloalkanes.

Question 12

What is the alkane substitution reaction?

Answer 12

When on hydrogen on an alkane is substituted with a halogen. In the presence of UV light (sunlight e.g) or at temperatures about 200C. Also known as free radical substitutions.

Alkane + Cl2 —(UV Light)—> Chloroalkane + HCl

Question 13

What is haloalkane substitution reactions? List all possible reactions.

Answer 13

When the polar bond between halogen and carbon can be substituted to form different reactions. Mainly occurs to primary haloalkanes as its easily reactive. Secondary and tertiary don’t react that often.

React with ammonia to form amine.
React with KCN to form nitrile.
React with halogen to form dihaloalkane.
React with NaOH to form alcohol.

Question 14

What is the reaction of haloalkanes to form nitriles, and to form alcohols?

Answer 14

Haloalkane —– KCN in ethanol + heat —–> Nitrile. (C-CN)

Haloalkane —– NaOH/KOH in ethanol + heat (100C) —–> Alcohol + NaCl (e.g)
This is very similar to the elimination reaction of haloalkanes. Note: Primary haloalkanes will mostly always do the substitution.

Question 15

What is the reaction of haloalkanes to form amines / and to form dihaloalkanes?

Answer 15

Haloalkane + Halogen2 —-UV Light—> Dihaloalkane + HCl.

Haloalkane + 2NH3 —-Ethanol solvent + heat—–> Amine + Ammoniumbromide e.g

Question 16

What are elimination reactions?

Answer 16

Reverse of addition reactions. Removing hydrogens to from the double bond again. Saturated –> Unsaturated. Strong base (NaOH) in ethanol and heat required. Water as a product of hydrogens and hydroxides.

Haloalkane + NaOH —> Alkene + NaBr + H2O

Question 17

What are oxidation reactions? List all possible oxidation reactions.

Answer 17

Defined as the loss of electrons, gaining of oxygen, or loss of hydrogen.
Oxidation of primary alcohol –> Aldehyde –> Carboxylic acid
Oxidation of secondary alcohol –> Ketone
(Also combustion reactions)
Combustion of alkanes
Tertiary has no hydrogens to lose so no oxidation possible.

Question 18

What is the oxidation of primary alcohols? And the conditions.

Answer 18

Loss of two hydrogen atoms, to form aldehyde. The alpha carbon increases in oxidation state.
Alcohol + (o) -> Aldehyde + H2O
Gain an oxygen atom to form carboxylic.

Require warm solution 50 C and more. K2Cr2O7 is acidified by H2SO4 for conditions.

Question 19

What is the oxidation of secondary alcohols + conditions?

Answer 19

Loss of two hydrogens, add a bond to oxygen to from ketone (carbonyl group).
Require warm solution 70 C and more. K2Cr2O7 is acidified by H2SO4 for conditions.

Question 20

What are reduction reactions? What are the conditions for them?

Answer 20

The gain of electrons, gain of hydrogens or loss of oxygen.
Occurs from Alkene to alkane and from nitrile to amine.
1. Hydrogenation
2. Nitriles reduced to primary amines, with LiAlH4 with water. Breaks triple bond to just single bond to NH2 and gives 2H to the carbon.