Chapter 13 Hydroxy Compounds

Question 1

Reaction of Alcohols With Acyl Chlorides

Answer 1

• Acyl chlorides are reactive organic compounds with a -COCl functional group
• The carbonyl carbon is electron-deficient and has a partial positive charge
• It is therefore susceptible to nucleophilic attack
  
  • The carbon-chlorine bond breaks and white fumes of hydrogen chloride, HCl, are formed

Question 2

Reaction with alcohols and phenols

Answer 2

• Acyl chlorides react with alcohols and phenols to form esters in a nucleophilic substitution reaction
• The -OH group acts as a nucleophile and attacks the carbonyl carbon to substitute the chlorine atom
• Forming esters from acyl chlorides rather than carboxylic acids is more effective because::
  
  • Acyl chlorides are more reactive (so it produces the ester faster)
  • Acyl chloride reactions go to completion (so more of the ester is produced)

Question 3

acly chlorides Reaction with alcohols

Answer 3

• The reaction of acyl chlorides with alcohols is vigorous and white fumes of HCl gas are formed

Question 4

Reaction with phenols

Answer 4

• For the reaction of acyl chlorides with phenols to occur, heat and a base are required
  
  • The base is needed to deprotonate the phenol and form a phenoxide ion
  • The phenoxide ion is a better nucleophile than the original phenol molecule and will be able to attack the carbonyl carbon

Question 5

Acyl chlorides react with phenols when heated and in the presence of a base to form esters

Answer 5

Question 6

A base is needed to form a phenoxide ion which is a better nucleophile than phenol; now, nucleophilic attack on the carbonyl carbon can more readily occur

Answer 6

Question 7

Phenols are

Answer 7

• organic compounds characterised by the presence of an -OH group which is attached to a benzene ring
• Phenols can be produced by the reaction of phenylamine with nitrous acid (HNO₂)

Question 8

Production of phenol

Answer 8

• Phenols can be prepared from phenylamines under the following reaction conditions:
  
  • NaNO₃ with dilute acid (to form HNO₂)
  • Ice to keep the temperature below 10 ^oC (step 1)
  • Heat (step 3)

Question 9

Production of phenol: This reaction involves three steps; Step 1

Answer 9

• Step 1 – The HNO₂ is so unstable that it needs to be prepared in a test-tube by reacting sodium nitrate (NaNO₃) and dilute hydrochloric acid (HCl) while keeping the temperature below 10 ^oC using ice

Question 10

Production of phenol: This reaction involves three steps; Step 2

Answer 10

• Step 2 – Phenyl amine is then reacted with the HNO₂ to form an unstable diazonium salt

Question 11

Production of phenol: This reaction involves three steps; Step 3

Answer 11

• • Step 3 – The diazonium salt is so unstable that it will thermally decompose when heated to form a phenol

Question 12

Reactions of Phenol

Answer 12

• Phenols can undergo many types of reactions as both the electron-rich benzene ring and the polar -OH group can participate in chemical reactions
• Some of the reactions of phenols include:
  
  • With bases
  • With reactive metals
  • With diazonium salts
  • Nitration
  • Bromination

Question 13

Reactions of Phenol: Reactions of the -OH group in phenols

Answer 13

• The -OH group in phenols has a slightly acidic character
• It can therefore act as an acid and take part in acid-base reactions

Question 14

Reactions of Phenol: Reaction with bases

Answer 14

• Phenols are only slightly soluble in water due to the large non-polar benzene ring
• However, they do dissolve in alkaline solutions and undergo acid-base reactions with bases to form a soluble salt and water

Question 15

Reactions of Phenol: Reaction with reactive metals

Answer 15

• Molten phenols react vigorously with reactive metals such as sodium (Na)
• This is also an acid-base reaction
• Now, a soluble salt is formed and hydrogen gas is given off

Question 16

Reactions of Phenol: Reaction with diazonium ions

Answer 16

• Diazonium ions are very reactive compounds containing an -N₂⁺ group
• When phenols are dissolved in sodium hydroxide (NaOH), a solution of sodium phenoxide is obtained
• This solution is cooled in ice and cold diazonium ion is added to the sodium phenoxide
• After the reaction has occurred, a yellow-orange solution or precipitate of an azo compound is formed
• These are compounds in which two benzene rings are linked by a nitrogen bridge

Question 17

Reactions of Phenol: Reactions of the aromatic ring in phenols

Answer 17

• Phenols react more readily with electrophiles compared to benzene
• This is because one of the lone pairs of electrons on the oxygen atom in -OH overlaps with the π bonding system
• This increases the electron density of the benzene ring making it more susceptible to electrophilic attack
• The -OH group in phenols is activating and directs incoming electrophiles to the 2, 4, and 6 positions

Question 18

Reactions of Phenol: Nitration

Answer 18

• Phenols can undergo electrophilic substitution reactions when reacted with dilute nitric acid (HNO₃) at room temperature and reflux to give a mixture of 2-nitrophenol and 4-nitrophenol
  
  • When concentrated HNO₃ is used, the product will be 2,4,6-trinitrophenol instead
• A hydrogen atom in the benzene ring is substituted by a nitro (-NO₂) group
• This is also known as the nitration of phenol

Question 19

Reactions of Phenol: Bromination

Answer 19

• Phenols also undergo electrophilic substitution reactions when reacted with bromine water at room temperature
• Phenol decolourises the orange bromine solution to form a white precipitate of 2,4,6-tribromophenol
• This is also known as the bromination of phenol

Question 20

Nitration & Bromination of Phenol

Answer 20

• Compared to benzene, phenol reacts more readily with electrophiles
• This is because one of the lone pairs of electrons on the oxygen atom in phenol overlaps with the π bonding system of the benzene ring
• As a result, there is now an increased electron density in the ring
• The electron-donating -OH group in phenol, therefore, activates the benzene ring and directs incoming electrophiles to the 2, 4, and 6 positions
• The increased reactivity of phenol means that different reagents and conditions are used for electrophilic substitution reactions of phenols compared to benzene

Question 21

Nitration

Answer 21

• Nitration is an example of an electrophilic substitution reaction
• The nitration of benzene requires a mixture of concentrated nitric acid (HNO₂) and sulfuric acid (H₂SO₄) refluxed with benzene between 25 ^oC and 60 ^oC
• Since phenol is more reactive, nitration can occur under milder conditions by reacting it with dilute nitric acid at room temperature
  
  • If concentrated nitric acid is used, 2,4,6-trinitrophenol is formed

Question 22

Bromination

Answer 22

• Bromination is another example of an electrophilic substitution reaction
• Benzene will undergo bromination only when reacted with pure bromine (not a solution) and in the presence of an anhydrous aluminium bromide (AlBr₃) catalyst at room temperature
• Phenol on the other hand readily reacts with bromine water in the absence of a catalyst

Question 23

Reagents & conditions for nitration and bromination of phenol & benzene table

Answer 23

Question 24

Directing Effects of Hydroxyl Group on Phenol

Answer 24

• Phenols consist of a hydroxyl (-OH) group attached to a benzene ring
• The oxygen atom in this hydroxyl group donates electron density into the ring
• One of the lone pairs of the oxygen atom overlaps with the π system of the benzene ring and become delocalised causing an increased electron density in the aromatic ring
• Due to the increased electron density, the benzene ring is now more likely to undergo electrophilic attack and becomes activated
• The incoming electrophiles are directed by the hydroxyl group of the phenol to the 2, 4, and 6 positions

Question 25

An example is the bromination of phenol

Answer 25

• The bromine acts as an electrophile and substitutes a hydrogen atom in the benzene ring
• The substitution of the hydrogen atom can occur on the 2, 4, or 6 positions

Question 26

Phenolic compounds

Answer 26

• are those that contain a phenol functional group

Question 27

An example of a phenolic compound is

Answer 27

1-naphthol

• 1-Naphthol contains a phenol group attached to another benzene ring
• Just like with phenol, the -OH group in 1-naphthol is also electron-donating and activates the benzene ring to electrophilic substitution reactions
• The electrophiles are directed to the 2 and/or 4 positions
• Substitution at the 6 position is not possible as there is no hydrogen atom on this carbon
  
  • This carbon is bonded to a carbon atom of the second benzene ring
• 1-Naphthol and other phenolic compounds react in a similar way as phenol