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A-level Chemistry AQA > 3.5 Alcohols > Flashcards

3.5 Alcohols Flashcards

1
Q

What is the functional group of an alcohol?

A

Hydroxyl group -OH

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2
Q

What is the general formula of an alcohol?

A

CnH2n+1OH

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3
Q

How do you name alcohols (one prefix, one suffix)?

A

Hydroxyl- or -OH

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4
Q

What kind of intermolecular forces do alcohols have? Why?

A

Hydrogen bonding, due to the electronegativity difference in the OH bond

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5
Q

How do alcohols’ m.p. And b.p. Compare to other hydrocarbons’ of similar C chain lengths? Why?

A

Higher, because they have hydrogen bonding → stronger than Van der Waals forces

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6
Q

Are alcohols soluble in water? Why does solubility depend on chain length?

A

Soluble when short chain - OH hydrogen bonds
to hydrogen bond in water
Insoluble when long chain - non-polarity of C-H
bond takes precedence

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7
Q

How can ethanol be made from crude oil?

A

Hydration of ethene via electrophilic addition (phosphoric acid catalyst H₃PO₄)

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8
Q

What are the advantages and disadvantages of hydration of ethene via electrophilic addition method?

A

Advantages - fast, continuous process, ethanol has a high purity
Disadvantages - not renewable as from crude oil

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9
Q

How can ethanol be made by fermentation?

A

Plant carbohydrates broken down and fermented by enzymes in yeast → ethanol

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10
Q

What conditions are needed for fermentation to take

place?

A

Enzymes in yeast as catalyst, 35°C, anaerobic conditions

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11
Q

What are the advantages and disadvantages of fermentation?

A

Advantages - renewable as from plants
Disadvantages - slow, batch process, enzymes stop working at 15% alcohol so solution is not pure, needed to be fractionally distilled

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12
Q

In the future, how might most ethene be made? Why is it not made like this at the moment?

A

Dehydrate ethanol made by fermentation → ethene

Not economical at the moment

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13
Q

Define carbon neutral

A

No net addition of CO2 to the atmosphere - carbon dioxide released when combusted = carbon dioxide absorbed as a plant

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14
Q

Explain how using ethanol in petrol engines could be considered to be carbon neutral.

A

Carbon dioxide released in fermentation and combustion = carbon dioxide absorbed when growing

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15
Q

Why would it probably not be entirely carbon neutral to use ethanol?

A

Other “carbon costs” associated with it e.g. Transport

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16
Q

What is a commercial fuel that uses ethanol? What else does it contain and why?

A

Methylated spirits - methanol (toxic, so it can’t be drunk)

17
Q

What is an elimination reaction?

A

The removal of a smaller molecule from a larger one

18
Q

Which group leaves the parent molecule in the case of alcohols?

A

OH and a H (to form water)

19
Q

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

A

Excess hot concentrated sulphuric acid or pass vapour over hot aluminium oxide

20
Q

What forms if you partially oxidise a primary alcohol?

A

Aldehyde

21
Q

What conditions are needed to partially oxidise a primary alcohol?

A

Dilute sulphuric acid, potassium dichromate (VI), distill product as it’s produced, gentle heating

22
Q

What forms if you fully oxidise a primary alcohol?

A

Carboxylic acid

23
Q

What conditions are needed to fully oxidise a primary alcohol?

A

Concentrated sulphuric acid, potassium dichromate (VI), reflux for about 20 mins, strong heating

24
Q

What forms if you oxidise a secondary alcohol?

A

Ketone

25
Q

Why can it not be oxidised further and why can’t a tertiary alcohol be oxidised?

A

A carbon-carbon bond would have to break

26
Q

What conditions are needed for the oxidation of a secondary alcohol?

A

Concentrated sulphuric acid, potassium dichromate (VI), reflux for about 20 mins, strong heating

27
Q

What does the Tollens’ test give a positive result for?

A

Aldehydes

28
Q

What is in Tollens’ reagent? How does this react with the substance to be tested?

A

Silver nitrate in NH3 (aq) - oxidises aldehydes but
not ketones

Complex silver (I) ions reduced to Ag(s)

29
Q

How do you carry out the Tollens’ test?

A

Add equal volumes of substance being tested and tollen’s reagent to a test tube, leave in water bath for 10mins and observe any changes

30
Q

What is the result of the Tollen’s test for aldehydes and ketones?

A

Aldehydes - silver mirror forms (solid Ag) Ketones - no visible change

31
Q

What does Fehling’s solution give a positive test result for?

A

Aldehydes

32
Q

What is in Fehling’s? How does this react with the substance to be tested?

A 
Blue copper (II) complex ions - gentle oxidising agent
Reduced to Cu+ ions (brick red)
33
Q

What conditions do you need to use the Fehling’s

solution?

A

Heat

34
Q

What result do aldehydes and ketones give in the Fehling’s test?

A

Aldehydes- brick red ppt

Ketones- no visible change

A-level Chemistry AQA (21 decks)

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