Ketone + alkene = reaction?

[SamuelTesla]

On question 98 of AAMC #7, there is a reaction I don't understand. A base abstracts an acidic alpha hydrogen, which gives the alpha carbon a negative charge. And then an alkene (which is relatively rich in electrons) adds to it (and during the process loses the double bond). What doesn't make sense to me is how two negatively charged compounds react together. Wouldn't a positive charge be attracted to the alpha carbon?

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[chemtopper]

Here you can see that negative carbon is going to get bonded with the positive end of the alkene bond

 

[SamuelTesla]

Here you can see that negative carbon is going to get bonded with the positive end of the alkene bond

thanks so much - i guess the way you show it makes sense - but I still don't understand why the middle carbon in propene is positively charged. Carbons donate electrons, so shouldn't it be negative (in addition to the fact that it is in a double bond that is electron rich)

 

[chemtopper]

thanks so much - i guess the way you show it makes sense - but I still don't understand why the middle carbon in propene is positively charged. Carbons donate electrons, so shouldn't it be negative (in addition to the fact that it is in a double bond that is electron rich)

Please have a good review of stability of carbocations .More substituted carbocations are more stable .SO +ve charge on secondary carbon is more stable than on primary carbon .you are right that alkyl groups are electron donating and that's the reason+ ve charge will go on carbon which has more alkyl groups on it.