Alcohols

Question 1

what shape are alcohols?

Answer 1

bent

Question 2

what 2 reactions do alcohols undergo?

Answer 2

• oxidation
• dehydration (elimination)

Question 3

what are alcohols water soluble?

Answer 3

due to dipole across O-H, therefore interact with water via hydrogen bonding

Question 4

how does solubility of alcohols change with chain length?

Answer 4

increased chain length decreases solubility - non-polar section gets bigger and polar O-H becomes less significant

Question 5

how does the MP and BP of alcohols compare to alkanes of similar Mr? why?

Answer 5

much higher- alcohols have hydrogen bonding due to OH

Question 6

how does MP and BP change with chain length? why?

Answer 6

increases with increased chain length- VDWs become stronger with increased Mr

Question 7

what are the 2 main ways that alcohols are produced?

Answer 7

• hydration of alkenes
• fermentation

Question 8

what alcohol is produced from fermentation?

Answer 8

ethanol

Question 9

what is alcohol produced from in fermentation?

Answer 9

glucose, C6H12O6

Question 10

what enzyme is used to produce ethanol in fermentation?

Answer 10

yeast

Question 11

equation for fermentation?

Answer 11

Question 12

why is ethanol as a result of fermentation known as a biofuel?

Answer 12

produced from glucose which comes from plants

Question 13

oxidising agents?

Answer 13

acidified potassium dichromate
acidified potassium manganate

Question 14

colour change when MnO4- is reduced?

Answer 14

purple to colourless

Question 15

colour change when Cr2O72- is reduced?

Answer 15

orange to green

Question 16

how to ensure oxidation of primary alcohol to aldehyde doesnt continue to c.acid?

Answer 16

use distillation

Question 17

what technique is used when a primary alcohol is oxidised to a carboxylic acid?

Answer 17

reflux

Question 18

what are secondary alcohols oxidised to?

Answer 18

ketones

Question 19

how to distinguish between aldehydes and ketones?

Answer 19

• Tollen’s Reagent
• Fehling’s solution

Question 20

how to make Tollens?

Answer 20

1. place approx 1cm3 of silver nitrate solution in a clean test tube
2. add 1 drop of NaOH to form a brown ppt of Ag2O
3. add dilute ammonia drop by drop until ppt dissolves - this is now tollens reagent [Ag(NH3)2]+

Question 21

how to test organic compound with tollens?

Answer 21

add a few drops of organic compound to tollens reagent and heat gently in a water bath. a silver mirror indicates a positive result (oxidation of aldehyde).

Question 22

what reactions occur when tollens reagent displays a positive result?

Answer 22

oxidation: aldehyde –> carboxylic acid
reduction: [Ag(NH3)2]+ + e- –> Ag(s) + 2NH3(aq)

Question 23

how to make Fehling’s solution?

Answer 23

mix Fehlings A+ B. forms blue solution containing Cu2+ ions

heat gently with organic compound

Question 24

what is the positive result for Fehling’s

Answer 24

blue solution –> red ppt

Question 25

what reactions occur when Fehling's displays a positive result?

Answer 25

oxidation: aldehyde---> carboxylic acid reduction: Cu2+ + 2OH- + 2e- ----> Cu2O + H2O

Question 26

what is the test for carboxylic acids?

Answer 26

add Na2CO3 (s) or (aq) to the test substance

Question 27

what is the positive result for carboxylic acids test?

Answer 27

effervescence caused by release of CO2(g) during neutralisation reaction. can be confirmed by running through limewater - will turn cloudy

Question 28

what type of reaction is the dehydration of alcohols?

Answer 28

elimination (H2O removed)

Question 29

what are the conditions for the dehydration of alcohols?

Answer 29

acid catalysed - conc H2SO4 or H3PO4 heated over Al2O3(s)