AS Chemistry - Unit 2

Cards on alcohols

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  • Created by: Kaimlee
  • Created on: 23-02-11 14:38

Alcohols

Alcohols contain the OH functional group, joint to a hydrocarbon chain.

General Formula - CnH2n+1OH

Shorthand written ROH (where R = an alkyl group)

Types of alcohols

Primary - The carbon atom joint to the OH is joint to no or just 1 carbon atom, i.e. methanol, ethanol...

Secondary - the carbon atom joint to the OH group is also joint to 2 other carbon atoms. i.e. propan-2-ol, 2-bromobutane...

Tertiary -  the carbon atom jonit to the OH group is also joint to 3 other carbon atoms. i.e. 2-methyl-2-propan-2-ol...

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Naming Alcohols

Naming:

CH3CH2CHOHCH3    =   butan-2-ol      (secondary)

CH2OHCH2OH           =  Ethan-1-2-diol

CH2OHCHOHCH2OH    = propan-1-2-3-triol

Molecules with OH groups and other functional groups are named by hydroxy.



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Reactions of Alcohols

Combustion: in excess oxygen alcohol burns to give carbon dioxide and water.

                 CH3OH + 1 1/2 O2 ---------> CO2 + 2H2O

                 C2H5OH + 3O2 -------------> 2CO2 + 3H2O

Oxidation

The oxidising agent is: Acidified Potassium Dichromate & Dilute Sulphuric Acid.

Colour change: Orange ----> Green

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Oxidation of Primary Alcohols

Oxidation of Primary Alcohols - 2 Stages

1.) Product a Aldehyde   CH3CH2OH + [O] ---> CH3CH=O + H2O

To gain this product - distill as forms (distilation)

                              - use limited oxidising agent

2.) Carboxylic Acid (e.g. ethanoic acid, propanoic acid...)

CH3CH2OH + [O] ---> CH3CH=O + H2O --> CH3COH=O

To gain this product - Heat under refux

                              - Use excess oxidising agent

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Oxidation of Secondary Alcohols

Secondary alcohols are oxidised into KETONES only - the carbon can only form 4 bonds, if the ketone is oxidised a C-C bond is broken which is very strong.

Heat under refux + oxidising agent -----> ketone + H2O

CH3CHOHCH3 + [O] -------> CH3C=OCH3 +H2O

Ketone =O within molecule (not on ends)

Tertiary Structure  alcohol can't be oxidised - the C-C bond most be broken - too strong

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Questions

Q1.) Write a balanced equation for the combustion of ethanol?

Q2.) What to products are formed from the combustion of methanol?

Q3.) Draw the structure (displayed formula) of a alcohol that can't be oxidised? name it? and  explain why?

Q4.) What is the general formula for an alcohol?

Q5.) When a primary alcohol is oxidised write an equation using [O] for the 2 possible products that could be formed? use: ethanol.

Q6.) What reagant is used for the oxidation of an alcohol?

Q7.) Describe how to gain only a carboxylic acid from the oxidation of a primary alcohol? and how to gain only a aldehyde?

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Test for Aldehydes or Ketone

Both have C=O group - carbonyl group     Reaction - gentle oxidation - an aldehyde will be gentle oxidised, but a ketone won't be.

Tollens' silver mirror test

Using: Tollens' reagant (silver nitrate & aqueous ammonia) and heating with unknown substance:

Oxidises an aldehyde - deposits of silver, but has no effect on a ketone

Fehlings/Benedicts test

Using: Fehlings reagant & benedicts reagant (both contain Cu blue ions) & heating, if an aldehyde colour change - blue ----> brick red precipitate (of CuO), no effect on ketone.

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Elimination Reaction - Alcohol

Products - Alkene + H2O

By 3 possible ways:

  • Pass the alcohol vapour over heated aluminum oxide (Al2O3)
  • Heat the alcohol with cold sulphuric acid
  • Heat the alcohol with concentrated phosphric acid

Dehydration: - removal of water

CH3CH2CH2OH ------------------->CH3CH=CH2 + H2O

                          Al2O3 + 600K
If OH group in the middle (e.g. CH3Ch2CHOHCH3) = a mixture of products. but-1-ene and (CH3)CH=CH(CH3) as E and Z isomers.

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Alcohol - Properties

Physical Properties:

Melting and boiling points are high for such small molecules.

Smaller alcohol molecules are more soluble in water, than the larger molecules.

Both these properties are caused by hydrogen bonding (an intermolecular force)




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Production of Ethanol

Method 1 - from ethene

  • Gain ethene from cracking crude oil
  • Ethene reacts with stream - catalyst: phosphoric acid (HPPO4)

Conditions: 200 (oC) & 65 x atm.pressure

  CH2=CH2 + 2H2O  -----------------------> C2H5OH

                                 Phosphoric Acid

Adv:                                  Dis:

Its a continous process       Rawl material (crude oil) non-renewable

produces pure ethanol        Process needs lots of energy

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Production of Ethanol

Method 2: Fermantation

Rawl Materials/Conditions: sugar (glucoses), H2O, yeast - enzymes

                      NO air (anareobic), time, Temperature 37 (oC)

Equation:

    C6H12O6  (aq) --------------------> 2C2H5OH (aq) + 2CO2 (g)

     glucose                                       ethanol

Adv:

  •  the ethanol produced is carbon neutral (CO2 produced balances that of photosynthesis)
  • Can be used as biofeul
  • more economical in the future

Dis:

  • The ethanol is v.unpure (only 10% pure)
  • Batch process (not contineous) taking 10-14 days (lots of time)
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Carbon Neutral - this means that the carbon dioxide released when it is burnt is balanced by the carbon dioxide absorbed by the plant from which it was originally obtained, during photosynthesis.

Ethanol most important alcohol - used in cosmetics, purfumes, drugs, ink...etc In industry ethanol/other alcohols are used as starting materials to make other organic chemicals

Q1.) State the 2 reagants used in the oxidation of a primary alcohol?

Q2.) The first oxidation product is propanal, under what conditions is this the major product?

Q3.) Write an equation using [O] to show this reaction?

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