In the OCR A2 textbook for chemistry A, the mechanism can be found on page 25 (if anyone wants to look it up).
Basically the reaction the mechanism shows is:
Propanal+ 2[H] -> Propan-1-ol
CH3CH2CHO + 2[H] -> CH3CH2CH2OH
Now these equations are fully balanced, with the H representing the reducing agent, which I easily understand.
Now, the mechanism shows:
The Hydride ion attacking the slightly positive carbon atom on the C=O bond, and forming a bond with this carbon, and at the same time the pi-bond breaking in the C=O bond so the oxygen has both these electrons and is negatively charged.
Now this is the part I don't understand - water decides to get involved...
So the negatively charged oxygen atom donates an electron pair to a hydrogen atom in a water molecule, forming a dative covalent bond, and a hydroxide ion. This completes the formation of the alcohol.
Now this equation no longer matches the one I stated earlier - now it is:
CH3CH2CHO + 2[H] -> CH3CH2CH2OH + OH-
i.e no longer balanced.
The only thing I could suggest to make this correct is that in the old equation they just simplified water to just a [H] so then could ignore the hydroxide ion.
So the above shoud have been:
CH3CH2CHO + [H] + H20 -> CH3CH2CH2OH + OH-
Anyway sorry about the length of this, would appreciate some clarification.