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Chemistry AS Level CIE > Chapter 17 Hydroxy compounds > Flashcards

Chapter 17 Hydroxy compounds Flashcards

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1
Q

Production of Alcohols

A
  • Alcohols are compounds that contain at least one hydroxy (-OH) group
  • The general formula of alcohols is CnH2n+1OH
  • Alcohols can be prepared by a wide range of chemical reactions
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2
Q

Production of Alcohol: Electrophilic addition of alkenes

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-When hot steam is reacted with an alkene, using concentrated phosphoric(VI) acid (H3PO4)as a catalyst, electrophilic addition takes place to form an alcohol

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3
Q

Production of Alcohol: Oxidation of alkenes

A

-Cold, dilute KMnO4 is a mild oxidising agent and oxidises alkenes

  • The C-C double bond is not fully broken and a diol is formed
  • -A diol is a compound with two hydroxy, -OH, groups
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4
Q

Production of Alcohol: Nucleophilic substitution of halogenoalkanes

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The halide atom in halogenoalkanes can be substituted when heated with aqueous NaOH in a nucleophilic substitution reaction

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5
Q

Production of Alcohol: Reduction of aldehyde & ketones

A

-Aldehydes and ketones can be reduced by reducing agents such as NaBH4 or LiAlH4

  • Aldehydes are reduced to primary alcohols
  • -The carbon attached to the hydroxy group is bonded to one other alkyl group
  • Ketones are reduced to secondary alcohols
  • -The carbon attached to the hydroxy group is bonded to two other alkyl groups
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6
Q

Production of Alcohol: Reduction of carboxylic acids

A

-
Similarly, carboxylic acids are reduced by NaBH4 or LiAlH4 to primary alcohols
-Carboxylic acids can also be reduced by H2 using a nickel catalyst and heat

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7
Q

Production of Alcohol: Hydrolysis of ester

A
  • Esters are made by a condensation reaction between an alcohol and a carboxylic acid
  • When an ester is heated with dilute acid or alkali, hydrolysis will take place and the carboxylic acid and alcohol will be reformed
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8
Q

Reactions of Alcohols: Combustion of alcohols

A
  • Alcohols react with oxygen in the air when ignited and undergo complete combustion to form carbon dioxide and water
  • Alcohol + oxygen → carbon dioxide + water
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9
Q

Reactions of Alcohols: Substitution of alcohols

A
  • In the substitution of alcohols, a hydroxy group (-OH) is replaced by a halogen to form an halogenoalkane
  • The substitution of the alcohol group for a halogen can be achieved by reacting the alcohol with:

–HX (rather than using HBr, KBr is reacted with H2SO4 or H3PO4 to make HBr that will then react with the alcohol)

–PCl3 and heat

–PCl5 at room temperature

–SOCl2

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10
Q

Substitution of alcohols to produce halogenoalkanes

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11
Q

Reaction of Alcohol: Reaction with Na

A
  • When an alcohol reacts with a reactive metal such as sodium (Na), the oxygen-hydrogen bond in the hydroxy group breaks
  • Though the reaction is less vigorous than sodium reacting with water, hydrogen gas is given off and a basic compound (alkoxide) is formed

–If the excess ethanol is evaporated off after the reaction a white crystalline solid of sodium alkoxide is left

Alcohol + sodium → sodium alkoxide + hydrogen

-The longer the hydrocarbon chain in the alcohol, the less vigorous the reaction becomes

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12
Q

Reaction of Alcohols: Dehydration of alcohols

A

-Alcohols can also undergo dehydration to form alkenes

–Dehydration is a reaction in which a water molecule is removed from a larger molecule

–A dehydration reaction is a type of elimination reaction

-Alcohol vapour is passed over a hot catalyst of aluminium oxide (Al2O3) powder OR pieces of porous pot or pumice as well as concentrated acid can be used as catalysts

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13
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14
Q

Reation of alcohol: Esterification of Alcohols

A
  • Esterification is a condensation reaction between a carboxylic acid and an alcohol to form an ester and a water molecule
  • For esterification to take place, the carboxylic acid and alcohol are heated under reflux with a strong acid catalyst (such as H2SO4 or H3PO4)

-Carboxylic acid + alcohol → ester + water

  • The reaction is reversible so an equilibrium mixture can be established with all the reactants and products
  • Esters have sweet, fruity smells
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15
Q

Reation of alcohols: Oxidation of alcohols with Acidified potassium dichromate(VI)

A

-K2Cr2O7, is an orange oxidising agent

  • Acidified means that that the potassium dichromate(VI) is in a solution of dilute acid (such as dilute sulfuric acid)
  • For potassium dichromate(VI) to act as an oxidising agent, it itself needs to be reduced
  • This reduction requires hydrogen (H+) ions which are provided by the acidic medium

–When alcohols are oxidised the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions

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16
Q

Reation of alcohols: Oxidation of alcohols with Acidified potassium manganate(VII)

A

-KMnO4, is a purple oxidising agent

  • As with acidified K2Cr2O7 the potassium manganate(VII) is in an acidic medium to allow reduction of potassium manganate(VII) to take place
  • When alcohols are oxidised, the purple manganate ions (MnO4–) are reduced to colourless Mn2+ ions
17
Q

Reaction of alcohols: oxidation summary

A
  • Primary alcohols can be oxidised to form aldehydes which can undergo further oxidation to form carboxylic acids
  • Secondary alcohols can be oxidised to form ketones only
  • Tertiary alcohols do not undergo oxidation
  • The oxidising agents of alcohols include acidified K2Cr2O7 or acidified KMnO4
  • Warm primary alcohol is added to the oxidising agent
  • The formed aldehyde has a lower boiling point than the alcohol reactant so it can be distilled off as soon as it forms
  • If the aldehyde is not distilled off, further refluxing with excess oxidising agent will oxidise it to a carboxylic acid
  • Since ketones cannot be further oxidised, the ketone product does not need to be distilled off straight away after it has been formed
18
Q

Classifying Alcohols

A
  • Primary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to one other carbon atom (or alkyl group)
  • Secondary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to two other carbon atoms (or alkyl groups)
  • Tertiary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to three other carbon atoms (or alkyl groups)
19
Q

which form of alcohols can get oxidised when mildly oxidised with acidified K2Cr2O7

A

-Only primary and secondary alcohols can get oxidised when mildly oxidised with acidified K2Cr2O7

–Primary alcohols get mildly oxidised to aldehydes

–Secondary alcohols get mildly oxidized to ketones

  • Tertiary alcohols do not undergo oxidation with acidified K2Cr2O7
  • Therefore, only the oxidation of primary and secondary alcohols will change the colour of K2Cr2O7 solution as the orange Cr2O72- ions are reduced to green Cr3+ ions
20
Q

Test for Alcohols

A

-Tri-iodomethane (also called iodoform) forms a yellow precipitate with methyl ketones

–Methyl ketones are compounds that have a CH3CO-group

–Ethanal also contains a CH3CO- group and therefore also forms a yellow precipitate with iodoform

  • The reagent is heated with an alkaline solution of iodine
  • This reaction involves a halogenation and hydrolysis step

–In the halogenation step, all three H-atoms in the -CH3 (methyl) group are replaced for iodine atoms, forming –CI3

–The intermediate compound is hydrolysed by alkaline solution to form a sodium salt (RCO2– Na+) and a yellow precipitate of CHI3

21
Q

The reaction of methyl ketones with iodoform results in the formation of a yellow CHI3 precipitate

A
22
Q

Tests for alcohol: Iodoform & alcohols

A
  • The position of a secondary alcohol can be deduced by reacting the compound with alkaline I2
  • If the -OH group is on the carbon atom next to a methyl group, it will firstly get oxidised to CH3CH(OH)- by the alkaline solution
  • This will result in the formation of a methyl ketone RCOCH3
  • The methyl ketone will then first get halogenated and then hydrolysed to form the sodium salt and the yellow precipitate
  • If no yellow precipitate is formed, then this means that the secondary alcohol is not on a carbon next to a methyl group
23
Q

Acidity of Alcohols

A
  • Alcohols have a low degree of dissociation
  • This means, that when dissolved in water, alcohol molecules do not dissociate (split up) to a great extent

ROH (aq) ⇄ RO– (aq) + H+ (aq)

Alcohol alkoxide ion

  • The position of the equilibrium lies to the left, meaning that there are far more alcohol molecules than RO– and H+ ions
  • When water dissociates, the position of the equilibrium still lies to the left, but there are more H+ ions compared to the dissociation of alcohols

H2O (l) ⇄ OH– (aq) + H+ (aq)

As alcohols have a lower [H+ (aq)] in solution compared to water, alcohols are weaker acids than water

24
Q

Acidity of Alcohols: The inductive effect in alcohols

A
  • Electron-donating species such as alkyl groups push electrons into a covalent bond and are said to have a positive inductive effect
  • In alcohols, the oxygen atom in the alkoxide ion is bonded to an electron-donating alkyl group
  • This means that there is more electron density on the O– atom
  • The alkoxide ion is, therefore, more likely to accept an H+ ion and form the alcohol again
  • When water dissociates, the hydroxide ion only has one other hydrogen atom
  • There is no extra electron density on the oxygen which is less likely to accept an H+ ion
  • Water is therefore a stronger acid than alcohols
25
Q

Water is a stronger acid than alcohols as there are no electron-donating groups in the hydroxide ion, causing the O– to be less likely to accept a proton and reform water

A
26
Q

Alkyl groups in the alkoxide ion donate electron density to the negatively charged oxygen, causing it to more readily accept a proton and form the alcohol again

A

Chemistry AS Level CIE (21 decks)

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