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AS Chemistry. Alcohols. Learning Objectives Candidates should be able to: recall the chemistry of alcohols, as exemplified by ethanol, including their oxidation to carbonyl compounds and carboxylic acids. classify hydroxy compounds into primary, secondary and tertiary alcohols.

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AS Chemistry

Alcohols


  • Learning Objectives

  • Candidates should be able to:

    • recall the chemistry of alcohols, as exemplified by ethanol, including their oxidation to carbonyl compounds and carboxylic acids.

    • classify hydroxy compounds into primary, secondary and tertiary alcohols.

    • suggest characteristic distinguishing reactions, e.g. mild oxidation.


Starter activity

Complete task 1 on your worksheet.


Production of alcohol


Classes of alcohols


Oxidation of alcohols

Primary alcohols

Secondaryalcohols


Oxidation of alcohols

Primary alcohol  aldehyde  carboxylic acid

e.g. ethanol ethanal ethanoic acid

Secondary alcohol  ketone  no reaction

e.g. propan-2-ol propanone

Tertiary alcohol  no reaction


Oxidising agent

Cr2O72- Cr3+

Orange  Green


Heating under reflux

Prevents evaporation of aldehyde


AS Chemistry

Physical properties and general chemistry


  • Learning Objectives

  • Candidates should be able to:

  • recall the chemistry of alcohols, exemplified by ethanol:

    • combustion

    • substitution to give halogenoalkanes

    • reaction with sodium

    • dehydration to alkenes


Starter activity

Complete task 1 on your worksheet.


Physical properties – boiling point


Physical properties – boiling point

  • The boiling point of an alcohol is always much higher than that of an alkane with similar Mr.

  • This is due to hydrogen bonding between the alcohol molecules.

  • In alkanes the only intermolecular forces are vdW.


Physical properties – solubility

The small alcohols are completely soluble in water.


Physical properties – solubility

In both pure water and pure ethanol the main intermolecular forces are hydrogen bonds. In order to mix them together you must supply energy to break these bonds.


Physical properties – solubility

  • Fortunately, alcohols can form hydrogen bonds with water:

    • H – OH

    • R - OH


Physical properties – solubility

This bond-making process releases energy which more or less compensates for the energy input.

N.B. an alcohol only has one + H atom, unlike water which has 2.


Physical properties – solubility

The hydrocarbon chains in an alcohol cannot form hydrogen bonds.

As the chains get longer they force their way between water molecules preventing them from hydrogen bonding.

Too little energy is released from bond-making to compensate for that used in bond-breaking.


Physical properties – boiling point

  • The boiling point of an alcohol is always much higher than that of an alkane with similar Mr.

  • This is due to hydrogen bonding between the alcohol molecules.

  • In alkanes the only intermolecular forces are vdW.


General chemistry – combustion

Ethanol can be used as a fuel, mainly as a biofuelalternative to gasoline. Because it is easy to manufacture and process and can be made from renewable resources such as sugar cane and corn, it is an increasingly common alternative to gasoline in some parts of the world, e.g. Brazil.

Anhydrous ethanol (ethanol with less than 1% water) can be blended with gasoline in varying quantities and most gasoline engines will operate well with mixtures of 10% ethanol (E10).


General Chemistry - electron density map

Ethanol – CH3CH2OH

Chemistry can involve breaking the C-O bond and the O-H bond.


General chemistry – reaction with Sodium

2Na + 2HO-H  2NaOH + H2

2Na + 2RO-H  2NaOR + H2

NaOR is more usually written as RONa or RO- Na+

2Na + 2CH3CH2OH  2CH3CH2O-Na+ + H2

sodiumethanolsodium ethoxide hydrogen


General chemistry – substitution to form halogenoalkanes

Consider the reaction below:

CH3CH2Br + OH- CH3CH2OH + Br-

You should remember that this hydrolysis reaction occurs quite readily in a warm, aqueous solution.

The reverse reaction is more difficult, -OH is not a good leaving group. Presence of H+ required to form water, a much better leaving group.


General equation

The general reaction looks like this:

ROH + HX  RX + H2O


Replacing –OH by bromine

Rather than using hydrobromic acid, you usually treat the alcohol with a mixture of sodium or potassium bromide and concentrated sulphuric acid.

This produces hydrogen bromide in situ which reacts with the alcohol. The mixture is warmed to distil off the bromoalkane (see page 338 of your textbook).


Replacing –OH by iodine

Iodoalkanes can be made by two different methods.

In the first method the alcohol is reacted with a mixture of sodium or potassium iodide and concentrated phosphoric(V) acid, H3PO4, and the iodoalkane is distilled off.


Replacing –OH by iodine

In the second method the iodoalkanecan be made by warming the alcohol with a mixture of red phosphorus and iodine:

This then reacts with the alcohol to give the corresponding halogenoalkane which can be distilled off.


Replacing –OH by chlorine

Chloroalkanes are more easilymade by the secondroute: you can react an alcohol with phosphorus(III) chloride, PCl3, phosphorus(V) chloride, PCl5, or sulphur dichloride oxide (thionyl chloride, SOCl2).

  • CH3CH2OH + PCl5 CH3CH2Cl + HCl + POCl3

  • CH3CH2OH + SOCl2 CH3CH2Cl + HCl + SO2


Elimination (when a small molecule is removed from a larger molecule – converts a single bond to a double bond)

C2H5OH(g) CH2=CH2(g) + H2O(g)


AS Chemistry

Aldehydes and Ketones


  • Learning Objectives

  • Candidates should be able to:

    • describe the reduction of aldehydes and ketones e.g. using NaBH4.

    • describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones.

    • describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect the presence of carbonyl compounds.

    • deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the result of simple tests (i.e. Fehling’s or Tollens’ reagents; ease of oxidation).

    • describe the formation of carboxylic acids from nitriles.


Starter activity

Can you write balanced equations for the synthesis of chloro-, bromo- and iodoethane from ethanol?

Give the names of the reagents and the reaction conditions.


Aldehyde or Ketone


Tollen’s reagent


Aldehyde or Ketone

Using Fehling's solution


Fehling’s reagent


Reduction of aldehydes and ketones – H2

butanone butan-2-ol

prop-2-enal  propan-1-ol


Reduction of aldehydes and ketones – NaBH4

ethanal ethanol

propanonepropan-2-ol


Reduction of aldehydes and ketones – NaBH4

prop-2-enal prop-2-en-1-ol


Formation of hydroxynitriles (or cyanohydrins)


Reactions of nitriles

Hydrolysis

Reduction


Aldehyde or ketone? 2,4-DNPH

Test:Add a solution of 2,4-dinitrophenylhydrazine (2,4-DNPH).

Result:a deep yellow or orange precipitate


AS Chemistry

Carboxylic acids


  • Learning Objectives

  • Candidates should be able to:

    • describe the reactions of carboxylic acids in the formation of salts.


Starter activity – can you complete task 1?

methanoic acid

2-methylbutanoic acid

hexanedioic acid


Acidity of the carboxylic acids

Reaction with metals


Neutralisation reactions

Reaction with alkalis

CH3COOH(aq) + NaOH(aq) CH3COO-Na+(aq) + H2O(l)

Reaction with carbonates

2H+(aq) + CO32-(aq)/(s) H2O(l) + CO2(g)

Reaction with hydrogencarbonates

H+(aq) + HCO3-(aq)/(s) H2O(l) + CO2(g)


AS Chemistry

Esters


  • Learning Objectives

  • Candidates should be able to:

    • describe the formation of esters from carboxylic acids using ethyl ethanoate as an example.

    • Describe the acid and base hydrolysis of esters.

    • State the commercial use of esters, e.g. solvents, perfumes, flavourings.


Naming esters

Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.


Naming esters

ethyl propanoate

propylethanoate

propylethanoate

butyl methanoate


Esterification and hydrolysis

c. H2SO4

warm

dil. H2SO4

reflux


Base hydrolysis

dilute

reflux

This reaction is irreversible !!

Note:  These reaction is exactly the reverse of those used to make an ester from a carboxylic acid and an alcohol. The only difference in that case is that you use a concentrated acid as the catalyst. To get as much ester as possible, you wouldn't add any water otherwise you would favour the hydrolysis reaction


Alkaline hydrolysis of fats or oils

Because of its relationship with soap making, the alkaline hydrolysis of fats and oils is sometimes known as saponification.


Uses of esters

Adhesives

Solvents

Fragrances

Flavourings


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