Alcohols - Organic Chem I

Question 1

What are alcohols governed by? Why are they present?

Answer 1

Strongest type of intermolecular force present: hydrogen bonds

Because the -OH group contains the two essential features needed: a hydrogen atom directly attached to a highly electronegative oxygen atom

Question 2

Why do alcohols have a higher mpt/bpt than alkanes of similar Mr value?

Answer 2

More energy is needed to overcome the London forces, diploe-dipole forces and hydrogen bonds between alcohol molecules than just the weak London forces between alkane molecules

Question 3

Why are alcohols (with short hydrocarbon chains) miscible with water?

Answer 3

Alcohols can form hydrogen bonds with molecules

Question 4

Why do diols, and particularly triols, have relatively high viscosity?

Answer 4

The greater the number of OH groups per molecule the stronger the hydrogen bond

Question 5

Show what the combustion of alcohol would look like?

Answer 5

Alcohols burn with a clean flame (implies complete combustion)
Eg/ CH3CH2CH2OH + 9/2O2 → 3CO2 + 4H2O

Question 6

What is the reagent, condition and product of the chlorination of alcohol?

Answer 6

Reagent: PCl5
Conditions: room temp
Product: chloroalkane

Question 7

What is the reagent, condition and product of the bromination of alcohol?

Answer 7

Reagent: KBr + 50% H2SO4
Conditions: heat under reflux
Product: bromoalkane

Question 8

What is the reagent, condition and product of the iodination of alcohol?

Answer 8

Reagent: I2 + moist red phosphorous
Conditions: warm
Product: iodoalkane

Question 9

What’s the test for alcohols?

Answer 9

Test: add a small amount of PCl5 to the substance
Result: if an alcohol is present steamy fumes are observed which turn damp blue litmus paper red (as HCl gas is produced)

Question 10

What is the equation of an alcohol reacting with PCl5?

Answer 10

ROH + PCl5 → RCl + POCl3 + HCl
Eg/ CH3CH2OH + PCl5 → CH3CH2Cl + POCl3 + HCl

Question 11

What does a dehydration reaction look like?

Answer 11

Removes H2O from an organic compound (alkene + H2O)
Reagents and conditions: conc H3PO4 (warm)

Question 12

What is the commonly use oxidising agent?

Answer 12

Potassium dichromate (VI) (K2Cr2O7) in dilute sulfuric acid

Question 13

Half equation for the reduction for Cr2O7 and give the colour change?

Answer 13

Cr2O7^2- + 14H+ + 6e- → 2Cr^3+ + 7H2O
Orange → green

Question 14

What is the alternative oxidising agent and give the half equation for that:

Answer 14

Potassium manganate (VII) in dilute sulphuric acid
MnO4- + 8H+ + 5e- → Mn2+ 4H2O
Purple → colourless

Question 15

What are primary alcohols oxidised to? Give the conditions

Answer 15

Primary alcohols are partially oxidised to aldehydes (by distillation) and fully oxidised to carboxylic acids (by heating under reflux) . Using potassium dichromate (VI) in dil. H2SO4

Question 16

Why is this reaction carried out in distillation apparatus?

Answer 16

Because the desired product, the aldehyde, has a lower boiling point than the primary alcohol. This means that as soon as the aldehyde forms it distils over preventing it from being further oxidised to the carboxylic acid. The primary alcohol remains in the flask, allowing more aldehyde to form

Question 17

Why does the aldehyde have a lower boiling point than the alcohol?

Answer 17

Aldehydes cannot form hydrogen bonds
Therefore less energy will be needed to overcome the London forces and dipole-dipole

Question 18

What does the oxidation of secondary alcohol look like?

Answer 18

Secondary alcohols are oxidised to ketones. Same oxidising agent is used and the mixture has to be heated under reflux

Question 19

What does heating under reflux allow you to do for a prolonged amount of time?

Answer 19

Heat a reaction for a prolonged amount of time without losing any reactants or products.

Question 20

What happens after the reaction mixture is heated?

Answer 20

The reaction mixture boils and the vapours travel up to the condenser, where they condense and drop back into the flask.

Question 21

How can the rate of boiling be controlled?

Answer 21

Using anti-bumping granules as well as monitoring the heat source.
- They encourage small bubbles to form and therefore smooth boiling

Question 22

Why does the water have to enter the condenser at the bottom?

Answer 22

So it flows through the water jacket against gravity to avoid any air bubbles. Ensures all of the condenser is cooled

Question 23

What are the two reasons in why you might want to use the distillation apparatus?

Answer 23

Can be used to obtain a pure product from a crude mixture. If the mixture contains only two or three components, then simple distillation apparatus can be used.

Question 24

What is the method of separating two solvent and product?

Answer 24

Solvent extraction - relies on the desired product being more soluble in the added solvent than the reaction mixture.

Question 25

What is observed when a drying agent is used?

Answer 25

The liquid should go from cloudy to clear (as the water is removed)

Question 26

Name the 3 main drying agents:

Answer 26

Anhydrous MgSO4(s)
Anhydrous Na2SO4(s)
Anhydrous CaCl2(s)

Question 27

What do you do to get rid of the drying agent?

Answer 27

The mixture is then filtered (decanted) to remove the drying agent

Question 28

What does it mean when a compound burns with a clean flame

Answer 28

The compound is saturated