[thewondermonkey]

If I have a 5-atom ring lactone with an aldehyde coming off the first carbon after the oxygen in the ring, which one is more reactive? My guess is the carbon of the lactone will be more electronegative because it has 3 bonds to electronegatives atoms as opposed to the aldehyde's 2, but the aldehyde also has much less steric strain.

Thoughts?

[thewondermonkey]

Just had another thought--I'm pretty sure the aldehyde is more reactive because of the resonance that forms among the electrons in the lactone. The ring-Oxygen actually inductively donates its negative charge to the partially positive carbon of the carbonyl, reducing its reactivity. Also, it is probably less reactive because of steric strain. Does that sound right?

[Babcock_Hall]

Reactivity toward what kind of chemical reaction?

[thewondermonkey]

Something simple like nucleophilic addition, I suppose. My homework then follows up with a question about what would form in a Wittig reaction with 1-chlorooctane.

[stewie griffin]

In general aldehydes are more reactive than ketones which are more reactive than esters/lactones. The ketone has 2 R groups, thus more electron donation to the carbonyl carbon whereas the aldehyde only has only electron donating R group. Esters/Lactones have an R group and the oxygen which reduces the positive character of the carbonyl carbon via resonance.
So depending on the reaction you want to do, you may be capable of reacting the aldehyde selectively. For example, a reduction with DIBAL at -78C will be selective for the aldehyde. A Wittig would go to the aldehyde. Sometimes it just takes running the reaction and finding out 

[Dan]

For example, a reduction with DIBAL at -78C will be selective for the aldehyde.

I think it will be selective for the lactone - DIBAL's an electrophilic reducing agent, it reacts faster with amides and esters than it does with aldehydes and ketones.

[stewie griffin]

Dan you caught me  . I was typing away and trying to come up with examples to help. My bad.