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WJEC Chemistry CH4 > Alcohols > Flashcards

Alcohols Flashcards

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1
Q

Give the trend in solubility of alcohol in water.

A 
Lower alcohols (short chain) are very soluble in water, being miscible in all proportions - attributed to the fact that alcohol can form H bonds with water molecules.
As C chain length increases, solubility decreases.
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2
Q

Why does solubility decrease as C chain length increases?

A

As the length of the non-polar hydrophobic alkyl chain increases, it becomes more difficult for the chain to penetrate into the hydrogen bonded structure.

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3
Q

Give the conditions needed to prepare alcohols from alkenes.

A

Direct Hydration (Industrial Preparation)

  1. Phosphoric (VI) acid on silica gel
  2. 300°C
  3. 60-70 atm

Hydration Using Conc. H2SO4 (Lab Preparation)

  1. Concentrated sulphuric (IV) acid / heat
  2. Hydrolysis (H2O/Reflux)
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4
Q

Give the reagents and conditions needed to prepare alcohols from halogenoalkanes and state the type of reaction.

A 
Nucleophilic Substitution 
Reagent: NaOH or KOH
Conditions:
1. Aqueous solvent
2. Room temperature or gentle warming to increase rate
3. Propanone as a mutual solvent
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5
Q

Give the reagents and conditions needed to prepare alcohols from carbonyl compounds, state the type of reaction and products formed.

A

Reduction (addition of hydrogen) using mild reducing agent NaBH4, sodium tetrahydridoborate (III) in aqueous solvent

Aldehydes are reduced to primary alcohols.
Ketones are reduced to secondary alcohols.

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6
Q

Give the reagents and conditions needed to prepare alcohols from carboxylic acids and state the type of reaction.

A

Reduction using

  1. powerful reducing agent LiAlH4, lithium tetrahydridoaluminate (III)
  2. in dry ether solvent
  3. Room temperature
  4. Followed by acid hydrolysis
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7
Q

How does alcohol react with sodium?

A

Alcohols react slowly to sodium to evolve hydrogen gas. This indicates acidic character. This reaction increases as C chain length increases.

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8
Q

Why are alcohols weaker acids than water and why do they become weaker as carbon chain length increases?

A

As the length of the C chain increases, the Inductive Effect increases. The O-H bond breaks less easily (weaker acid).

There is no inductive effect present in water so the O-H bond breaks homolytically to release proton.

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9
Q

What is the Inductive Effect?

A

Alkyl groups are electron-donating; they push electrons away from themselves.
This increases the electron density in the O-H group, strengthening the bond.

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10
Q

Give the conditions required to partially oxidise primary alcohols into aldehydes.

A

Partial Oxidation

Oxidising agent:

  1. Potassium dichromate (VII)
  2. Sulphuric (VI) acid (2 moldm-3)

Conditions:

  1. Excess alcohol (limits further oxidation)
  2. Distil out aldehyde as it forms
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11
Q

Give the conditions required to fully oxidise primary alcohols into carboxylic acids.

A

Full Oxidation

Oxidising agent:

  1. Potassium dichromate (VII)
  2. Sulphuric (VI) acid (2 moldm-3)

Conditions:

  1. Excess oxidising agent (ensures max. yield)
  2. Reflux
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12
Q

What does a secondary alcohol oxidise to?

A

Ketone

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13
Q

Colour change involving dichromate?

A

Orange to dark green

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14
Q

Tertiary alcohols show no reaction with acidified dichromate solution. Why?

A

Oxidation of a tertiary alcohol would require the breaking of a C-CH bond, resulting in a fragmentation of the molecule. This does not occur with moderately strong oxidising agents.

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15
Q

How do you convert an alcohol back to an alkene?

A

Heat with a dehydrating agent such as concentrated sulphuric (VI) acid or phosphoric (V) acid.

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16
Q

Why are tertiary alcohols more easily dehydrated than secondary and why are secondary more easily dehydrated than primary alcohols?

A
  • Reaction involves formation of carbocation intermediate
  • Increasing stability of carbocation intermediate from primary to tertiary
  • Lower activation energy hill + faster reaction
  • Less drastic conditions needed
17
Q

Why does alcohol not undergo nucleophilic substitution with Cl- or Br-?

A

The OH- group of the alcohol is highly basic and is a very poor leaving group. Hence, it is not displaced by the attacking nucleophile.

18
Q

How does an alcohol molecule become more susceptible to nucleophilic attack?

A

In acid condition, initial protonation of the OH group renders the OH group far less basic and hence, the molecule becomes more susceptible to nucleophilic attack.
Here, a neutral water molecule is a better leaving group so that substitution can be achieved by refluxing the alcohol with HBr.

19
Q

State the type of reaction and the conditions needed to convert alcohols to esters by reaction with carboxylic acids.

A

Type of reaction: Nucleophilic Condensation

Conditions:

  1. Sulphuric (VI) acid catalyst (or HCl)
  2. Reflux
  3. Anhydrous ZnCl2 (absorbs water as it forms, pushing equilibrium to the right, increasing yield of ester formed)
20
Q

What is meant by Nucleophilic Condensation?

A
  • It is addition followed by elimination.
  • Initial attack is by a nucleophile, giving an addition product.
  • Eventually, a small molecule (often H2O or HCl) is eliminated at a later stage in the mechanism.
21
Q

What is a nucleophile?

A

Species possessing a lone pair of electrons that is capable of forming a dative covalent bond to an electropositive centre.

22
Q

Why does esterification occur easily when using acyl chlorides/acid chlorides?

A
  • Acyl chlorides are much more reactive than carboxylic acids to nucleophilic attack.
  • Chlorine atom creates a highly electropositive carbon centre
  • allowing for nucleophilic attack at room temperature
23
Q

Why is the method in making esters with acyl chlorides preferred than with carboxylic acids?

A

The reaction does not involve an equilibrium and can give a much better yield of ester.

24
Q

State the type of reaction and the steps needed to convert alcohols to esters by reaction with acyl chlorides/acid chlorides.

A

Type of reaction: Nucleophilic Condensation

Steps:

1) Carboxylic acid is first converted into the acyl chloride using PCl5, Phosphorus (V) chloride at room temperature.
2) Add the alcohol ‘drop wise’ with cooling in an ice water bath.

25
Q

Describe the Iodoform Reaction.

A

Used to test for a particular molecular environment.

Reagents: Iodine and sodium hydroxide solution, warm

Positive test: Yellow crystals of iodoform, CHI3 will form on cooling.

26
Q

What happens when carbon chain length increases?

A
  • mpt and bpt increases
  • size of electron cloud increases
  • hence strength of intermolecular VDW forces increase

WJEC Chemistry CH4 (8 decks)

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