Module 4 (chapter 14, 15, 16) - organic chemistry

Question 1

functional group of alcohols

Answer 1

• OH hydroxyl group

- responsible for both the physical and chemical properties of the alcohols

Question 2

uses of methanol and ethanol

Answer 2

• methanol is used as a high performance fuel because of its efficient combustion. Its also used in chemical feedstock and other useful products like paint
• ethanol is used primarily in alcoholic drinks and as a fuel

Question 3

differences between alcohols and the corresponding alkanes

Answer 3

• alcohols are less volatile
• have higher melting points
• greater water solubility

Question 4

why are alcohols less volatile and have higher melting points than alkanes?

Answer 4

• in the liquid state, intermolecular hydrogen bonds hold the alcohol molecules together and must be broken to change the liquid into a gas
• this requires more energy that overcoming the weak London forces in alkanes

Question 5

why are alcohols more soluble in water than alkanes?

Answer 5

A compound that can form hydrogen bonds with water is far more water soluble than a compound that cannot

• alkanes are non-polar molecules and cannot form hydrogen bonds
• whereas alcohols can form bonds between the polar -OH group of the alcohol and the water molecules
• yet as the hydrocarbon chain increases in size the influence of the OH group becomes relatively smaller and the solubility decreases

Question 6

primary alcohols

Answer 6

the -OH group is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group

Question 7

secondary alcohols

Answer 7

the -OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups

Question 8

Tertiary alcohols

Answer 8

the -OH group is attached to a carbon atom that is attached to no hydrogen atoms and rather three alkyl groups

Question 9

combustion of alcohols

Answer 9

alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water

• the reaction is exothermic, releasing a large quality of energy in the form of heat
• as the number of carbon atoms increases the quantity of heat released per mole also increases.

Question 10

oxidation of alcohols

Answer 10

primary and secondary alcohols can be oxidised by an oxidising agent

• the usually oxidising mixture is potassium dichromate acidified with dilute sulfuric acid
• if the alcohol is oxidised the orange solution containing dichromate ions is reduced to a green solution containing chromium ions

Question 11

what can primary alcohols be oxidised to form?

Answer 11

either aldehydes or carboxylic acids

Question 12

how do you prepare an aldehyde from a primary alcohol?

Answer 12

• gentle heating of primary alcohols with acidified potassium dichromate forms an aldehyde.
• gently heating only as aldehydes have no hydrogen bonds so a lower boiling point
• to ensure an aldehyde is formed it is distilled out of the reaction mixture as it forms
• this prevents further reaction with the oxidising agent

Question 13

how do you prepare a carboxylic acid from a primary alcohol?

Answer 13

• if heated strongly under reflux with an excess of acidified potassium dichromate a carboxylic acid is formed
• excess acidified potassium dichromate is used to ensure that all of the alcohol is oxidised
• heating under reflux ensure any aldehyde formed initially in a reaction also undergo oxidation to the carboxylic acid

Question 14

what happens in the oxidation of secondary alcohols?

Answer 14

• oxidised to form ketones
• to ensure the reaction goes to completion the secondary alcohol is heated user reflux with the oxidising mixture
• the dichromate ions once again change colour from orange to green
• distill secondary alcohols because ketones have lower boiling points than alcohols due to no hydrogen bonds

Question 15

oxidation of tertiary alcohols?

Answer 15

do not undergo oxidation

-acidified potassium dichromate remains orange

Question 16

when distilling alcohols what mitigation techniques are necessary?

Answer 16

• clamp the apparatus
• remove the funnel and replace with a bung or thermometer to help prevent loss of product and it evaporates
• the direction of water flow must ensure it goes up instead of down in order to effectively cool the product

Question 17

what is a dehydration reaction?

Answer 17

any reaction in which a water molecule is removed from the starting material

Question 18

dehydration of alcohols

Answer 18

an alcohol is heated under reflux in the presence of an acid catalyst to form an alkene
-this is an example of an elimination reaction

Question 19

substation of alcohols

Answer 19

alcohols react with hydrogen halides to form haloalkanes

• for this, the alcohol is heated under reflux with sulphuric acid and a sodium halide the hydrogen bromide is formed in situ (in place)
• the hydrogen bromide reacts with the alcohol to produce a haloalkane

Question 20

reactivity of the haloalkanes

Answer 20

haloalkanes have a carbon-halogen bond in their structure

• halogen atoms are more electronegative than carbon atoms meaning the electron pair in this bond sits closer to the halogen atom
• this forms a polar bond
• this delta positively charged carbon atom can attract species containing a lone pair of electrons (nucleophiles)
• when a haloalkane reacts with a nucleophile it replaces the halogen in a substitution reaction

Question 21

nucleophilic substitution in the haloalkanes

Answer 21

-primary haloalkanes undergo nucleophilic substitution reactions with a variety of different nucleophiles to produce a range of compounds

Question 22

hydrolysis

Answer 22

a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule, resulting in it being split into two products
-in a hydrolysis reaction the atom is replaced by an -OH group

Question 23

first three stages of haloalkane hydrolysis

Answer 23

• the nucleophile (OH-) approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
• the direction of attack by the OH- ion minimises repulsion between the nucleophile and the (delta -) halogen atom
• a lone pair of electrons on the hydroxide ion is attracted and donated to the (delta +) carbon atom

Question 24

last three stages of haloalkane hydrolysis

Answer 24

• a new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
• the carbon-halogen bond breaks by heterolytic fission
• the new organic product is an alcohol
• a halide ion also forms

Question 25

conditions for conversion of a haloalkane into an alcohol through hydrolysis

Answer 25

- haloalkanes can be converted into alcohols using aqueous sodium hydroxide. the reaction is very slow at room temperature so the mixture is heated under reflux to obtain a good yield of product - forms a sodium halide

Question 26

nucleophile

Answer 26

- an electron pair doner | - it is an atom or group of atoms attracted to an electron deficient centre

Question 27

carbon-halogen bond strength

Answer 27

- the rate of hydrolysis depends on the relative strength of the carbon halogen bond - the CF bond is much stronger than the CI bond so less energy is required to break it leading to a faster rate of hydrolysis - this makes fluroalkanes nearly unreactive.

Question 28

measuring the rate of hydrolysis of primary haloalkanes

Answer 28

rate can be determined by carrying the reaction out in the presence of aqueous silver nitrate -as the reaction takes place, halide ions are produced which react with silver ions to form a precipitate of silver halide

Question 29

how to combat insolubility of haloalkanes in water

Answer 29

carry out in the presence of an ethanol solvent | -ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers.

Question 30

organohaloalkanes compounds

Answer 30

molecules that contain at least one halogen atom joined in a carbon chain - they are used as pesticides - rarely found in nature and as don't natural break down in the environment have become a point of concern

Question 31

the ozone layer

Answer 31

- found at outer edge of the stratosphere (varies between 10-40Km above the earth's surface) - only a small proportion of the gases are ozone but this is enough to absorb most of the biological damaging UV radiation (UV-B) from the sun allowing only a small amount to reach earth - its feared that continual depletion of the ozone layer will allow move UV-B radiator to reach the earth's surface - bad news for living organisms linked to increased genetic damage and a greater risk of skin cancer

Question 32

how is ozone made?

Answer 32

- continually being formed and broken down by action of UV radiation - initially very high energy UV breaks oxygen molecules into oxygen radicals, O: - O2 --> 2O - a steady state is then set up involving O2 and the oxygen radicals in which ozone forms and then breaks down. in this steady state, the rate of formation of ozone is the same as the rate at which it is broken down. - O2 + O O3

Question 33

what were CFCs initially used for?

Answer 33

- most common compounds used as refrigerants, aerosols propellents and air conditioning units - CFCs are very stable because of the strength of the carbon halogen bond within their molecules

Question 34

what did Rowland and Molina conclude?

Answer 34

- looked at the effect of CFCs on the earth's atmosphere - concluded that they remained stable until they reach the stratosphere where they begin to break down forming chlorine radicals - there are though to catalyse the breakdown of the ozone layer

Question 35

how do CFCs deplete the ozone layer?

Answer 35

- the stability of CFCs due to the strength of the carbon halogen bond means they have a long residence time in the troposphere - it may take them many years to reach the stratosphere - once here, UV radiation provides sufficient energy to break a carbon-halogen bond by homolytic fission to form radicals - the C-Cl bond has the lowest bond enthalpy and so is the bond that breaks

Question 36

what is the process of CFC's breaking down called?

Answer 36

as radiation initiates the breakdown this process is called photodissociation (e. g. CF2Cl2 --> CF2Cl. + Cl.) - the chlorine radical created is a very reactive intermediate it can react with the ozone molecule breaking down the ozone into oxygen

Question 37

two propagation stems of breakdown of oxygen

Answer 37

-step 1: (Cl. + O3 --> ClO. + O2) -step 2: (ClO. + O --> Cl. + O2) overall equation: O3 + O --> 2O2 - the two propagation stages repeat in a cycle over and over again in a chain reaction - it has been estimated that a single CFC molecule can promote the breakdown of 100,000 molecules of ozone

Question 38

are all CFCs responsible for the break down of ozone layer?

Answer 38

- other radicals also catalyse the breakdown of ozone | - nitrogen oxide radicals are formed naturally during lightening strikes and also aircraft travel in the stratosphere

Question 39

nitrogen radicals breaking the ozone layer down

Answer 39

- nitrogen oxide radicals cause the breakdown of ozone by a similar mechanism - step 1: NO. + O3 --> NO2 + O2 - step 2: NO2. + O --> NO. + O2 - overall equation is the same as the chlorine radicals