the C=O bond is so reactive due to its polarity. despite this it is stronger than a C=C bond. the C=O bond is also shorter than a C-C or C=C bond although this does not change its reactivity much compared to the effects of the polarity.
aldehydes and ketones generally react in the same sort of ways. the top three reaction types are nucleophilic addition, α substitutions and carbonyl-carbonyl condensations
the angle of attack is determined by the maximum orbital overlap between the lone pairs of the nucleophile and the π* anti bonding orbital on the carbonyl C. the addition of the electrons to the antibonding orbitals means that there are more electrons in the antibonding orbitals than the bonding ones and so the bond breaks.
the most optimal angle is 90˚ to the double bond but because of oxygens repulsive effect to the already electron dense nucleophile, it approaches at 107˚, this is called the Bürgi-Dunitz angle.
Because $\ce{CN-}$ is a good leaving group, it is able to reversibly bind to the the carbonyl carbon. the C=O bond can reform from the intermediate alkoxides ion as it is more stable. cyanide can still be used in nucleophilic addition reactions but the alkoxides intermediate must be worked up with acid to form the alcohol which prevets the reforming of the carbonyl group
nucleophilic additions occur because the bond enthalpy of C-Nu and O-H have a lower combined energy than the C=O π bond. this drives the reaction forward :)
an acid catalyst is normally used during this reaction as protonation of the oxygen makes for a much more electrophilic carbon as the bond is more polarised by a negative charge on the oxygen
nucleophiles without a charge (water, alcohols, amines and thiols) are able to protonate the alkoxide ions by them self without the need for acid conditions. this is shown in the mechanism below
the nucleophile is protonated during addition to the carbon. this extra proton is taken by the free nucleophile group to form an acid which can then transfer the proton to the alkoxide oxygen
generally aldehydes react faster than ketones in nucleophilic addition reactions. this is because the approach of the nucleophile is hindered by the two R groups either side of the carbon. the ketone R groups also work to reduce the partial positive charge via inductive donation of electrons across the bond to the δ+ carbon
to reduce an aldehyde or ketone you must first react it with a hydride ion to form an alkoxide ion and then with an acid to protonate that negative oxygen
this is a standard nucleophilic addition reaction
becuase aluminum is more electropositive than boron, the H-Al bonds are more easily broken. this allow for a faster rate of reaction when using $\ce{LiAlH4}$ over $\ce{NaBH4}$.