alcohols - runnymedechemistry.com€¦ · primary alcohols just as with haloalkanes, in a primary...

Alcohols

General Formula: Cn H 2n+1OH

Draw and name the isomers when n = 5:

Production

Fermentation

From alkenes

Note: H-bonding increases mpts and bpts, and increases solubility in water. Smallchain alcohols H-bonding is more significant. As the chain lengthens, the “aliphatic”part of the molecule begins to dominate. Acetone is an excellent solvent for thelonger chain alcohols as is is a polar organic solvent (it can H-Bond!!).

Test for the –OH (hydroxyl) group*:

C2H5OH + PCl5 C2H5Cl + POCl3 + HCl

Or, more generally:

R-OH + PCl5 R-Cl + POCl3 + HCl

*in alcohols and carboxylic acids.

Phosphorus pentachloride produces white steamy fumes of HCl informing an alkyl chloride.

The reaction vessel must be perfectly dry,as water also reacts to give steamy fumes –try the equation:

Primary alcoholsJust as with haloalkanes, in a primary (1°) alcohol, the carbon which carries the -OHgroup is only attached to one alkyl group and 2 (or 3) H atoms:

Some examples of primary alcohols include:

Secondary alcoholsThe adjacent carbon is attached to two alkyl groups. Examples:

Tertiary alcoholsThe adjacent carbon atom is attached directly to three alkyl groups.Examples:

Reactions of alcohols With: K2Cr2O7(aq) Conc. H2SO4

NaBr or KBr P and I

With acidified K2Cr2O7 (aq)This is oxidation, and produces aldehydes, ketones and carboxylic acids (used todistinguish between 1º, 2º and 3º alcohols).

In organic chemistry, we use simplified versions of redox reactions.An oxidising agent is represented as [O].

*aldehydes and ketones have the general formula CnH2nO – they contain a C=O, aldehydes at the end of thechain and ketones somewhere along it. See later.

Primary alcohols can be oxidised first to aldehydes* and then to carboxylic acidsdepending on the reaction conditions.

Partial oxidation to aldehydesExcess alcohol, and distil off the aldehyde as soon as it forms (volatile).

The excess of the alcohol means that there isn't enough oxidising agent present tocarry out the second stage.

Ethanol produces the aldehyde CH3CHO - Ethanal

What's happening...

The second stage:

Overall...

Full oxidation to carboxylic acidsExcess oxidising agent and heat under reflux. When the reaction is complete, thecarboxylic acid is distilled off.

The reaction has nowhere to go, as there is no H atom on the C=O to slip another [O] in.

Secondary alcohols are oxidised to ketones*

Eg. Heat propan-2-ol with potassium dichromate(VI) solution acidified with dilutesulphuric acid…

This reaction can be used as a test to distinguish between 1º, 2º and 3º

Tertiary alcoholsTertiary alcohols cannot be oxidised with potassium dichromate(VI) solution.

The carbon-oxygen double bond cannot be formed as the 2H atoms asociated with the C atomdo not exist:

The concentrated sulphuric acid is a catalyst.

Alternatively...

With conc. H2SO4 - dehydration (also using Al2O3 catalyst – cracking, GCSE).

The alcohol is heated with an excess of concentrated sulphuric acid at a temperature of170°C.

The ethene is collected over water.

*We used PCl5 to insert Cl

With NaBr or KBr and conc. H2SO4

To replace the –OH with Br*

Add to the alcohol a mixture of sodium or potassium bromide and concentratedsulphuric acid.

This produces hydrogen bromide (in situ) which reacts with the alcohol. Themixture is warmed to distil off the bromoalkane

Eg.

With P and I - Replacing -OH by iodine

Moist red phosphorus and iodine are heated with the alcohol (again, to prepare the PI insitu).

2P + 3I2 2PI3

PI3 + 3ROH H3PO3 + 3RI

Eg. How would you prepare 1-iodopropane?

Nomenclature of aldehydes, ketones and carboxylic acids

NB. We number from the end closest to the C=O

Context Studies: Green Chemistry and the Pharmaceutical Industry

Tasks: Spider diagram of all of the organic reactions. Project: Liquid and gaseous fuels

Synthetic pathways The reactions we have covered are fundamental in the synthesising of desiredmolecules in organic chemistry. Perhaps nature will provide us with a similar moleculeto the one required, and we manipulate it to meet our needs.

NB. Reaction of Alcohols with Sodium

Questions p211