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Please note that the half term test will take place during week beginning 12th October. This will consist of a set of short answer structured questions that will be answered on the test paper. The content will be based on coverage so far, but do remember that you will have been reading the OCR book too... ...!
These notes were originally put onto the A2 site...but you might as well hear them here! Click on the "posted by" line...
The haloalkanes represent an extremely versatile group of molecules which would be well worth taking with you onto your desert island...should you ever find yourself marooned. The polar bond introduced with the appearance of the halogen atom gives the carbon atom a delta-positive charge...and so invites the attention of a nucleophile, which is quick to effect a substitution reaction so charactersitic of saturated substrates.
The most polar may well be the C-Cl bond, but the most reactive is the C-I, since the latter is the longest and therefore the weakest. This fact is easily proven by the rate of the reaction with hydroxide ion, releasing the halide ion to react with aqueous silver (I) ion and give a ppt of silver iodide.
Remember that the haloalkanes may be prepared by reaction of alkenes with halogen or hydrogen halide [electrophilic addition] or from alkanes via a free radical substitution type of reaction.
Alcohols can suffer dehydration reactions as well as oxidation. However, an important difference is that ALL classes of alcohols, including tertairy, can be deydrated, although of course tertiary ones cannot be oxidised.
The reagent is concentrated sulphuric acid, but the conditions become milder as the classification moves from primary to secondary to tertiary. For example, ethanol is dehydrated with 95% sulphuric acid at 170 deg. to give ethene; butan-2-ol with 60% sulphuric acid at 100 deg and methylpropan-2-ol with 20% sulphuric acid at 85-90 deg.
Remember that in each case the product will be an alkene, but with some alcohols different isomeric alkenes are possible. For simplicity's sake consider the -OH group being removed together with a hydrogen atom from a neighbouring carbon atom, to make water. This can give the unsaturation either side of the original alcohol group, with geometric isomerism also a possibility, depending on the structure of the original alcohol. Do watch out for this!
These compounds contain the -OH functional group. There is a school of thought that includes water in this category, since this too contains the -OH group. I leave it to you to decide whether to subscribe to this ideal, but you can guess my conclusion!
They are often prepared by a nucleophilic substitution reaction on a haloalkane. The reagent for this is often aqueous sodium hydroxide, and the reaction mixture is heated under reflux.
Alcohols will burn, but this destructive oxidation is of no use...unless you happen to want to enjoy the energy stored in the bonds to cook on.
Oxidation is best described as the addition of oxygen or the removal of hydrogen [as water]. These statements describe exactly what happens to the molecules.
Oxidation that preserves the carbon skeleton is effected using potassium dichromate (VI) in dilute sulphuric acid. This is a delightful orange colour, and if it does its job it will end up green.
Alcohols are classified as primary, secondary or tertiary. This is determined by looking at the alcoholic carbon atom and counting how many other carbon atoms this is bonded to...and making the connection! That's fine except for methanol..but this is always classified as primary.
Oxidation of primary alcohols yields firstly the aldehyde and then subsequently the carboxylic acid. To stop at the aldehyde, add the oxidant to the warm alcohol and distill out the aldehyde as quickly as it is formed. The acid is obtained by arranging to heat the mixture under reflux.
Secondary alcohols will give ketones. Tertiary alcohols are resistant since the alcoholic carbon atom does not carry the hydrogen atom necessary for the first step of the reaction.