alcohols + haloalkanes + analysis

Question 1

what are the 3 types of alcohol

Answer 1

primary - OH group attached to a C atom bonded to 1 alkyl group
secondary - OH group attached to a C atom bonded to 2 alkyl groups
tertiary - OH group attached to a C atom bonded to 3 alkyl groups

Question 2

explain why alcohols are water soluble

Answer 2

alcohols are polar due to their OH group, and are able to form hydrogen bonds with water, as well as permanent dipole dipoles and london forces, which allows the alcohol molecules to pack closely together with water molecules and dissolve

Question 3

give 4 physical properties of alcohols

Answer 3

• ability to form hydrogen bonds
• low volatility
• high boiling points
• high solubility in water

Question 4

explain why alcohols have high boiling points

Answer 4

alcohols are polar and have OH groups, allowing them to form hydrogen bonds as well as permanent dipole dipole and london forces, which holds the molecules closely together and so more energy is needed to overcome + boil the substance

Question 5

explain why alcohols have low volatility

Answer 5

alcohols have OH groups which allows them to form hydrogen bonds as well as permanent dipole dipole and london forces, and lots of energy is needed to overcome these strong intermolecular forces holding molecules together, so alcohols do not change state easily

Question 6

how does chain length change the properties of alcohols

Answer 6

as the chain length increases, alcohols become a lot more similar to alkanes
e.g. becoming less soluble in water or decreased boiling point

Question 7

how does the number of OH groups change the properties of alcohols

Answer 7

multiple OH groups makes the differences between alkanes and alcohols more distinct
e.g. it increases water solubility and boiling point

Question 8

summarise 6 main reactions of alcohols

Answer 8

• complete combustion to produce H2O + CO2
• dehydration via elimination under reflux with conc H2SO4 catalyst to make alkenes
• halogenation via nucleophilic substitution with NaX + H2SO4 catalyst under reflux to make haloalkanes
• oxidation of primary alcohols with K2Cr2O7 + H2SO4 catalyst under distillation to produce aldehydes
• oxidation of primary alcohols with K2Cr2O7 + H2SO4 catalyst under reflux to produce carboxylic acids
• oxidation of secondary alcohols with K2Cr2O7 + H2SO4 catalyst under reflux to produce ketones

Question 9

outline the difference in conditions when oxidising a primary alcohol to aldehydes and carboxylic acids

Answer 9

to form an aldehyde - gentle heating under distillation with acidified potassium dichromate

to form a carboxylic acid - strong heating under reflux with excess acidified potassium dichromate

Question 10

what will you observe when oxidising an alcohol with acidified potassium dichromate

Answer 10

a colourchange from orange to green

Question 11

what happens when oxidising a tertiary alcohol

Answer 11

no reaction occurs
this is not possible - as the OH group is attached to a C atom bonded to 3 alkyl groups, no H can be removed so oxidation cannot occur

Question 12

what are the 3 types of haloalkane

Answer 12

primary - halide group attached to a C atom bonded to 1 alkyl group
secondary - halide group attached to a C atom bonded to 2 alkyl groups
tertiary - halide group attached to a C atom bonded to 3 alkyl groups

Question 13

explain why haloalkanes are more reactive than alkanes

Answer 13

halogens are very electronegative, and as there is an electronegativity difference between the halogens and carbon C-X bonds are polar, which creates a slight positive charge on the carbon, allowing it to attract nucleophiles

Question 14

outline 3 main reactions of haloalkanes

Answer 14

• hydrolysis via nucleophilic substitution with NaOH under reflux to produce alcohols
• nucleophilic substitution with NaCN + H2SO4 to produce nitriles
• nucleophilic substitution with NH3 + ethanol + NaOH to produce amines

Question 15

give 4 uses of organic halogen compounds + an example

Answer 15

• refrigerants
• flame retardants
• solvents
• used for making polymers

e.g. CFCs - chlorofluorocarbons

Question 16

why are CFCs environmentally unfriendly

Answer 16

build up of CFCs in the atmosphere causes the breakdown of ozone, as they encounter UV light from the sun causing the formation of Cl radicals, which catalyse the breakdown of the ozone layer from O3 to O2, causing gaps which mean more UV radiation reaches earth
- this can increase risk of skin cancer and sun burn

Question 17

what is the formula for ozone + give the equilibrium equation for the formation of ozone in the atmosphere

Answer 17

ozone = O3

O2 + O <> O3

Question 18

outline the steps for the breakdown of ozone by CFCs (CF2Cl2) + give the overall equation

Answer 18

let * = radical
CF2Cl2 >UV> CF2Cl + Cl
Cl* + O3&raquo_space; *ClO + O2
ClO + O&raquo_space; Cl + O2

overall reaction is
O3 + O&raquo_space; 2O2

Question 19

outline the role of Cl* in the breakdown of the ozone

Answer 19

Cl* is a catalyst, as it is regenerated in propagation step 2

Question 20

what alternatives are used now instead of CFCs

Answer 20

HFCs - hydrofluorocarbons are now used, as they do not contain Cl so free radicals aren’t formed so they are environmentally friendly

Question 21

outline a chemical test to identify chloroethane, bromoethane and iodoethane

Answer 21

• add 2cm3 ethanol to 3 test tubes and heat in a water bath to around 60C
• add a few drops of each haloalkane to separate test tubes
• follow this with 1cm3 of AgNO3(aq) solution
• immediately start a stop watch after this step and record observations + times
  POSITIVE= a precipitate should form in each one, white for chloroethane, cream for bromoethane, yellow for iodoethane
  the yellow precipitate will form first, then the cream one, then the white one - this is because C-I bonds break faster than C-Br bonds, which break faster than C-Cl bonds due to bond enthalpies

Question 22

explain the difference between reflux and distillation

Answer 22

reflux is the continuous evaporation and condensation so volatile compounds cannot escape, whereas distillation is evaporation then condensation allowing the most volatile compounds to escape/distil out

Question 23

how does IR spectroscopy work

Answer 23

covalent bonds absorb IR radiation, this causes them to vibrate
different bonds absorb different IR frequencies, so absorptions can be used to detect a range of functional groups
- for IR radiation to be absorbed the bond but have a permanent dipole, and to be IR active, its polarity must change as a result of the vibration of the bonds - this means diatomic molecules, which have no electronegativity difference, are not IR active e.g. Cl2, H2 etc

Question 24

explain how atmospheric gases can be linked to global warming

Answer 24

earth absorbs UV from the sun and re emits it away from the earth’s surface
greenhouse gases e.g. CO2, CH4, H2O also absorb IR radiation and trap it as heat which circulated in the atmosphere or re emits it back to earth surface, causing global warming

Question 25

what 2 other uses does IR spectroscopy have

Answer 25

- it can be used to monitor gases causing air pollution e.g. CO emissions from cars - it is used in modern breathalysers to measure ethanol levels in breath - high absorbance of IR radiation = high amounts of alcohol in breath

Question 26

give 3 uses of mass spectroscopy

Answer 26

- identifying isotpes of an atom - finding relative atomic/formula masses of molecules - finding molecular structure

Question 27

what is the M+1 peak

Answer 27

this is a smaller peak due to the natural abundance of 13C isotope this may be larger for molecules with more C atoms