Alcohols

What is the functional group of an alcohol?

Hydroxyl group -OH

What is the general formula of an alcohol?

CnH2n+1OH

How do you name alcohols?

Use suffix -ol (or -diol/-triol)

If another functional group, use prefix hydroxy-

How do you classify alcohols?

By the number of alkyl groups attached to the carbon bearing the -OH

1-primary

2-secondary

3-tertiary

What kind of intermolecular forces do alcohols have and why?

Hydrogen bonding due to the electronegativity difference in the OH bond

How do alcohols mp and bp compare to other hydrocarbons of similar carbon chain lengths?

They have higher bp/mp due to hydrogen bonding

Whats the trend in bp/mp for alcohols?

• Bp increases with chain length - more VdWs forces

• Bp decreases with branching - fewer contact points

Are alcohols soluble in water?

Yes because they form hydrogen bonds with water

What’s the trend in solubility for alcohols?

Decreases as chain length increases as the hydrophobic alkyl chain dominates (become insoluble at octonol)

How can ethanol be made from crude oil? What are the conditions?

Hydration of ethene via electrophilic addition

H3PO4 catalyst

High temperature (300C) and high pressure (60 atm)

What are advantages and disadvantages of making ethanol via hydration of alkenes?

Advantages:

Fast, continuous process, pure product

Disadvantages:

Non-renewable as from crude oil

How can ethanol be made by fermentation? What conditions are needed?

Plant carbohydrates broken down and fermented by enzymes in yeast to produce ethanol solution

Enzymes in yeast as catalyst

35C temp

Anaerobic conditions - no oxygen

Write an equation for the fermentation reaction which produces ethanol

C6H12O6 (aq) + (yeast cat.) → 2C2H5OH (aq) + 2CO2 (g)

What are the advantages and disadvantages of producing ethanol via fermentation of glucose?

Advantages:

Renewable

Disadvantages:

Slow, batch process, solution isn’t pure

Define carbon neutral

No net addition of CO2 to the atmosphere

CO2 taken in during a process = CO2 released during a process

Explain how using ethanol in petrol engines could be considered to be carbon neutral. Why would it probably not be entirely carbon neutral?

CO2 released in fermentation and combustion = CO2 absorbed during photosynthesis

However, other “carbon costs” associated with it eg transport and harvesting

What is an elimination reaction?

The removal of a smaller molecule from a larger one

What do alcohols lose in an elimination reaction?

OH from alcohol group

H attached to alcohol C

Overall = H2O

What conditions are needed for the elimination reactions from alcohols to alkenes? (2 alternatives)

1) Acid catalysed elimination:

Conc H2SO4/H3PO4 catalyst

Heat under reflux

2) Surface catalysed elimination

Pass alcohol vapour over Al2O3 catalyst

300C temp

Draw a mechanism for the dehydration/elimination of ethanol

What forms if you partially oxidise a primary alcohol?

An aldehyde

What forms if you fully oxidise a primary alcohol?

A carboxylic acid

What conditions are needed to partially oxidise a primary alcohol?

• Distillation

• DILUTE H2SO4

• Potassium dichromate (VI)

• Gentle heating

What conditions are needed to fully oxidise a primary alcohol?

• Reflux

• Strong heating

• CONC H2SO4

• Excess potassium dichromate (VI)

What is reflux used for?

Ensures completion of a reaction by allowing you to heat a mixture for a long time at the boiling point of the chemicals without losing any volatile reactants or products

Write an equation for the partial oxidation of ethanol

CH3CH2OH + [O] → CH3CHO + H2O

Write an equation for the full oxidation of ethanol

CH3CH2OH (l) + 2[O] → CH3COOH + H2O

What forms if you oxidise a secondary alcohol?

A ketone

Write an equation for the oxidation of propan-2-ol (secondary alcohol)

CH3CH(OH)H3(l) + [O] → CH3COCH3(g) + H2O(l)

Why cant tertiary alcohols be oxidised?

The carbon attached to the -OH group has no hydrogen atoms on it

How would you test for an alcohol? What would a positive result show?

For primary or secondary alcohols:

-acidified K2Cr2O7 + H2SO4 and WARM

-positive result - orange → green