F322: Alcohols

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Making And Using Alcohols

Hydration Of Ethene

H2C=CH2 + H2O ---H3PO4 / 300C / 60 atm ---> CH3CH2OH

The reaction is reversible, and so converstion of ethene is comcomplete; so they pass through over and over up to 95% conversion.


C6H12O6 --- Yeast / 37C ---> 2CH3CH2OH + 2CO2

-Ethanol produced can only reach 14% concentration as at this point the yeast doesn't work.
Also has to be undertaken in a vaccum to reduce incomplete conversion.

-Ethanol can be used in drinks, having to be distilled for higher percentages.
-It can also be as an additive to fuel to increase the octave rating.
-Methylated spirits consists of partially ethanol; used as a fuel.
-Methanol can be used as a clean burning fuel, and as a feedstock.

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Properties Of Alcohols

-Hydrogen bonds form between adjacent alcohol molecules.

-The volatility and boiling point is affected by these strong bonds compared to other alkanes (etc.), so more energy is needed to break these intermolecular forces.

-Alcohols can dissolve in water as H bonds form between the polar -O-H groups in alcohol and water.
-Solubility decreases with increase in alcohol size as a larger part of the molecule is the carbon chain; and these do not form hydrogen bonds. 

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Classification Of Alcohols

Primary Alcohol

-The -OH group is attached to a C atom with no alkyl groups, or one alkyl group.
-E.g. Methanol

Secondary Alcohol

-The -OH group is attatched to a C atom bonded to two alkyl groups.
-E.g. Propan-2-ol

Tertiary Alcohol

-The -OH group is attached to a C atom bonded to three alkyl groups.
-E.g. 2-methylbutan-2-ol

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Combustion Of Alcohols

In a plentiful supply of oxygen, alcohols will burn completely:
-C2H5OH + 3O2 -> 2CO2 + 3H2O 

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Oxidation Of Alcohols

Primary Alcohols
-C3H7OH + [O] --- K2Cr2O7 / H2SO4 / Distil ---> C2H5CHO + H2O
-This produces a aldehyde and water. (I.e. Propanal) The distillation prevents total conversion into a carboxylic acid.

-With excessive oxidising agent, the reaction will reach the carboxylic acid stage; and during lab work you will need to reflux the mixture so it is fully reacted.
-C3H7OH + 2[O] ---K2Cr2O7 / H2SO4 / Reflux ---> C2H5COOH + H2O

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Oxidation Of Alcohols (Cont.)

Seconday Alcohols
-These become oxidised into ketones, which cannot be oxidised further.
-C4H9OH + [O] --- K2Cr2O7 / H2SO4 / Heat ---> CH3C=OCH2CH3 + H2O

Tertiary Alcohols
-Resistant to oxidation and do NOT react.

Test for Primary/Seconday/Tertiary alcohols:

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-Esterification is the reaction of an alcohol with a carboxylic acid to form an ester and water.
-Requires a concentrated acid catalyst, such as sulfur dioxide.

-This is shown in the diagram as the removal of water to form an ester.

-Esters are used as adhesives, solvents and flavourings.
To prepare an ester:
-Add 1cm3 of the carboxylic acid and the alcohol.
-Place in a water bath of 80 degrees for 5 minutes.
-Pour the product into cold water, forming a sweet smelling oil. 

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Dehydration Of Alcohols

-Dehydration is an elimination reaction in which water is removed from a saturated molecule to form an unsaturated molecule.
-A concentrated H2SO4 catalyst is required and a large amount of heat.
-The alcohol must be under reflux for around 40 minutes, to eliminate all water.

This works for cyclic alcohols also:

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