4.2 Alcohols,haloalkanes and Analysis

Question 1

properties of alcohols

Answer 1

• saturated homologous series and has the general formula cnH2n+1OH
• The OH is polar so interacts with water molecules via hydrogen bond therefore alcohols are soluble in water. as the hydrocarbon chain increases, the solubility of the alcohol decreases as the aliphatic chain is non-polar so cannot form hydrogen bonds and this becomes the largest part of the molecule.
• High boiling point than corresponding alkane as they form hydrogen bonds with each other which is the strongest intermolecular forces so are volatile

Question 2

how are alcohols classified

Answer 2

• primary - the carbon atom bonded to the “-OH” group is bonded to one other carbon
• secondary- the carbon atom bonded to the -“OH” group is bonded to two other carbons
• tertiary- the carbon atom bonded to the “-OH” group is bonded to 3 other carbon atoms.

Question 3

combustion of alcohols

Answer 3

• alcohols are fuels and are used to generate energy. they burn readily with light blue flame forming CO2 and H20.

Question 4

how are alcohols oxidised?

Answer 4

• using acidified potassium dichromate which is a mild oxidising agent so is reduced(gains electrons) itself

Question 5

oxidation of primary alcohols

Answer 5

• primary alcohols can be oxidised to aldehydes via distillation then carboxylic acid via reflux
  colour changes in both reactions from orange to green.

Question 6

oxidation of secondary and tertiary alcohols

Answer 6

• secondary alcohols are heated under reflux to form a ketones. colour change from orange to green
• tertiary alcohols cannot be oxidised using dichromate only burning them so remains orange.

Question 7

dehydration of alcohols

Answer 7

• a chemical reaction is which water is lost so is a elimination reaction forming a alkene and water molecule.
• alcohol is heated with a strong acid catalyst either sulphuric acid (H2S04) or phosphoric acid (H3PO4)
  -E/Z isomers can be formed and two different products depending on which carbon the hydrogen is removed from to form water

Question 8

making haloalkanes

Answer 8

• substitution reaction made when alcohol is reacted with a halide ion using a acid catalyst e.g. sulfuric acid (H2SO4)
  formula: ROH + HX = RX + H20

Question 9

hydrolysis of haloalkanes in a substitution reaction

Answer 9

• There is a large difference in the electronegativities of carbon and halogens therefore haloalkanes are polar. the halogen being partially negative.
• the partially positive carbon atom is attacked by nucleophiles
• can form alcohols as it is a nucleophile (
  Nucleophilic substitution)

Question 10

nucleophile

Answer 10

an electron pair donor

Question 11

explanation of the trend in the rates of hydrolysis of primary haloalkanes in terms of the bond enthalpies of carbon–halogen bonds

Answer 11

-down the halogen group the bond enthalpies decreases. This is because the atom gets larger son are further away from the nuclei and experience stronger shielding. meaning further down the group the c-x bond are easier to break and are more likely to undergo nucleophilic substitution.

Question 12

compare experimentally the rates of hydrolysis of different carbon–
halogen bonds

Answer 12

1) place the haloalkanes into test tubes
2) add silver nitrate solution ethanol (solvent) to each tube
- yellow iodide precipitate forms first
- cream bromide precipitate second
- white chloride precipitate last

Question 13

CFCS (chlorofluorocarbons)

Answer 13

• CfCs are broken down by UV radiation forming Cl radicals which catalyse the breakdown of ozone.
• N02 also reacts in the same way from vehicle emissions and thunder storms

Question 14

benefits of CFCs

Answer 14

• they are stable, nonreactive and non-toxic chemicals used in fridges as a refrigerant, and as a propellant in deodorants

Question 15

scientific evidence of CFCs damaging the ozone layer

Answer 15

• following the montreal protocol to ban the use of CFCs in most products and temporary alternatives are used, evidence showed the ozone layer is closing up again.

Question 16

how does infrared spectroscopy work

Answer 16

• infrared (IR) radiaton causes covalent bonds to absorb energy causing them to vibrate
• each bond vibrates at its own frequency
• the absorbed energies are displayed on the infrared spectrum.

Question 17

the suspected link to global warming and resulting changes to energy usage

Answer 17

• C=O,O-H and C-H absorb infrared radiation. The bonds are present in many greenhouse gases such as CO2, H20 and CH4 which are released when fuel are burned.
• when greenhouse gases are released infrared radiation is adsorbed leading to artificial warming of the planet (global warming).
• suspected link has led to government including policies that encourage the use of technologies and energy resources that do not release greenhouse gases into the environment.

Question 18

Where do most organic compounds produces peaks atom a infrared spectrum?

Answer 18

-3000cm-1 due to absorption by C-H bonds

Question 19

How can a carboxylic acid on the infrared spectrum be identified

Answer 19

• C=O bond and the broad peak of the O-H bond

Question 20

Use of mass spectrum of an organic compound to identify molecular peak

Answer 20

• vaporised molecules are bombared with high energy electrons (ionisation)
• mostly 1 electron is knocked off leaving molecules with a +charge and a •radical
• molecular ion is detected
• lost electron is negligible so the molecular mass of the molecular ion = the the relative molecular mass of the compound.

Question 21

What is the m+1 peak caused by?

Answer 21

-The small proportion of carbon-13