Reactivity of aldehydes and ketones

before we get into the reactivity aldehydes and ketones let's first review the bonding in a carbonyl so carbonyl is the carbon double bonded to the oxygen so let's focus in on this carbon right here on the formaldehyde molecule let's find the hybridization state of this carbons I'm going to draw an arrow to this and to find the hybridization state one way to do is to think about the steric number all right when the steric number is number of Sigma bonds plus number of lone pairs of electrons so to that carbon let's count up some Sigma bonds here so we have a sigma bond to this hydrogen a single bond to this hydrogen and in our double bond here one of those is a sigma bond and one of those is a pi bond so we have a total of three sigma bonds so three sigma bonds and zero lone pairs of electrons gives us a steric number of three which we know means must have three hybrid orbitals and so this carbon is sp2 hybridized so if i'm going to go ahead and draw that carbon over here so that carbon is sp2 hybridized which means that it has three sp2 hybrid orbitals and go ahead and put those three sp2 hybrid orbitals in here like that alright and we know that carbon has unhybridized p orbitals i'm going to go ahead and draw in that unhybridized p orbital right here next let's let's think about those hydrogen's right so these hydrogen's go ahead and put them in red here so these hydrogen's right here bonded to that carbonyl carbon alright those have an electron s orbital which it was spherically shaped so I can put an S orbital in here and the overlap of course would be a sigma bond right so I have those Sigma bonds right there next let's look at the hybridization of the carbonyl oxygen here so the same idea number of Sigma bonds plus number of lone pairs of electrons so there's one Sigma bond between the oxygen and the carbon and then we have two lone pairs of electrons so one sigma bond and two lone pairs of electrons gives a steric number of three which means that oxygen must be sp2 hybridized as well so the oxygen has three sp2 hybrid orbitals so let me go ahead and draw those so put in the oxygen right here the oxygen has three sp2 hybrid orbitals I'm going to go ahead and draw in those sp2 hybrid orbitals so there's one and then I have these two over here and so the lone pairs of electrons on the oxygen right one lone pair is going to go into this sp2 hybrid orbital the other lone pair is going to go into this sp2 hybrid orbital and and then we have an overlap right here for this carbon so that's of course the Sigma bond between the carbon and the oxygen if the oxygen is sp2 hybridized it must also have an unhybridized p orbitals image drawn the unhybridized p orbital on the oxygen here like that and then we can see that the PI bond right comes from the overlap the side-by-side overlap of our P orbitals and so let me go and highlight the PI bond over here so in that double bond again on the dot structure for formaldehyde one of those is a sigma bond and one of those is a PI bond and over here we can see it on the right so this represents the bonding of the of the carbonyl and that's going to be important we think about things like molecular geometry all right if the carbon is sp2 hybridized all right then we know then we know that these atoms lie on the same plane here and we have bond angles close to a hundred and twenty degrees and so we'll talk more about that in a few minutes next let's think about the the polarization of that carbonyl alright so once again we look at the let's go down to this generic aldehyde here and then we have this carbonyl carbon attached to this oxygen oxygen is more electronegative than carbon so it's going to withdraw some electron density and we show the polarization with this arrow here alright the arrow points in the direction of the electrons right so the electron is going to be pulled closer to the oxygen and so the oxygen is going to get a little bit partially negative so we draw a partial negative sign here the oxygen is withdrawing some electron density from my carbonyl carbon right here so my carbonyl carbon is going to be partially positive like that and for an aldehyde right we know that alkyl groups are electron donating all right so this R group right here on the left you can think about that as being a little bit electron donating which means this R group is going to donate some electron density hmmm so I'll draw an arrow showing that some electron density is being donated by the R group so remember when we did carbo cations and we put alkyl groups on our carbo cations the more alkyl groups we had the more the carbo cation was stabilized so here we have one R group donating a little bit of electron density attempting to stabilize that partially positive charge on the on the carbonyl carbon let's go over here to the ketone and we have a similar situation right we have once again the oxygen withdrawing some electron density from our carbonyl carbon so we have a partial negative on our oxygen and our carbonyl carbon is a partial positive but this time we have to our groups so this R group on the Left can donate some electron density this R group on the right can donate some electron density and once again think about the carbo cations right the more alkyl groups we have the more our full positive charge and our carbo cation was stabilized similar idea here right the more R groups you have the more you stabilize the partial positive charge on your carbonyl and so because of that ketones are a little bit more stable than aldehydes just thinking about the polarization so there's more of a polarization in an aldehyde carbonyl than in a ketone alright so now let's let's put these these ideas together let's think about a nucleophilic addition reaction to a carbonyl and so I'm going to go ahead and draw I'm gonna draw a ketone down here and I know that the geometry around the carbon on a ketone is trigonal planar right because we talked about the bonding already so I'm going to go ahead and draw this in so we're going to have let's make an R prime group coming out at us in space and an R group going away from us in space and we know that these all are on the same plane like that so I'm drawing in my plane here alright we also know that there is a polarization of the carbonyl alright so the oxygen is more negative and the carbon is a little bit positive like that so the carbon is partially positive that means that once electrons it's electrophilic and that's extremely important when you're talking about reactions right nucleophilic additions to carbonyls so a nucleophile is going to come along so let's go ahead and draw in a nucleophile here so let's go ahead and make it a negatively charged nucleophile so like that so the nucleophile is going to be attracted to positive things right opposite charges attract and so the nucleophile is going to attack the partially positive carbon eel carbon like that and when it does so it's going to form a bond and therefore kick off these PI electrons here off onto this oxygen so let's go ahead and draw the result of our nucleophilic attack here so we're going to show a bond form between the carbon and the nucleophile all right so let me go ahead and highlight those electrons here all right so these electrons right here on the nucleophile have now formed the bond between the nucleophile and the carbon and see we still have our oxygen or oxygen used to have two lone pairs of electrons but it picked up another lone pair of electrons so a negative 1 formal charge now and then we have our R group so let's go ahead and draw our R groups over here so we have our prime and then we have our over here like that and so so be an intermediate and if we think about the geometry of this carbon let me go ahead and label it here so the geometry of this carbon now if I calculate the steric number I have four sigma bonds and so four Sigma bonds means a steric number four four hybrid orbitals so this carbon must be sp3 hybridized now so an sp3 hybridized carbon and so therefore your bond angle is closer to 109 degrees so in particular we're going to think about these R groups here and so this angle right in here somewhere is somewhere around 109 degrees so I'm going to write that approximately 109 degrees and so the bond angles change alright so go back over here to to this situation on the Left right this carbon right here was sp2 hybridized alright and therefore everything was planar and so these bond angles were approximately equal at approximately 120 degrees and so we've gone from approximately a hundred twenty degrees to a hundred and nine degrees for our intermediate here so the over here on the right our sp3 hybridized carbon has tetrahedral geometry so we call this our tetrahedral intermediate so let me go ahead and write that this is our tetrahedral intermediate and it's an alkoxide anion and so now we can we can compare aldehydes ketones in terms of reactivity and so the first factor is the polarization of the carbonyl so we've already seen that aldehydes are more polarized than ketones and so therefore the carbonyl carbon is a little bit more positive and that means the nucleophile can attack that positive charge a little bit more and so that's one reason why aldehydes are more reactive than ketones the polarization of the carbonyl another reason has to do with steric hindrance so when you think about when you think about a ketone alright and let's say we have some big bulky R groups here on this ketone so those bulky R groups might prevent the nucleophile from attacking so it turns out there's an optimum angle for the nucleophile to attack the carbonyl carbon and if you have bulky R groups they might prevent that they also would interfere with the formation of the tetrahedral intermediate because if I had big bulky R groups right I'm changing the bond angle from 120 degrees approximately 120 degrees over here on the left to 109 degrees those bulky R groups have to get closer together in space and they would of course repel and so there's some steric hindrance upon formation of your tetrahedral intermediate as well and so for those for those two reasons we we observe aldehydes to be more reactive than ketones