hey it's professor Dave, let's talk about hemiacetals, acetals, and imines
so let's take a look at some interesting aqueous equilibria that
we can see. let's say we have this aldehyde right here, now what can
happen is this
carbonyl oxygen can protonate and so there we have that right there and now
let's say we have some alcohol in solution, we've got this aldehyde in
acidic conditions and we've got some alcohol, well this alcohol certainly can
attack right here because this carbonyl carbon is very electrophilic because it
also has this formal positive charge on the oxygen, that oxygen really
wants to become neutralized, this alcohol can attack right there
this pi bond can go ahead and neutralize that oxygen so now we've got
that right here and now some molecule of solvent like another
molecule of
alcohol let's say can go ahead and grab that proton, we don't really care what
it is, in general something is gonna grab that proton and we're gonna get to the
hemiacetal. so this is called a hemiacetal because this is the hemiacetal
carbon it is a carbon that is bound to OR and OH, R being any alkyl so
O and more alkyl and then OH, if both of those are on one carbon that is called a
hemiacetal carbon but it doesn't have to stop at the hemiacetal because
let's say this hydroxyl goes ahead and protonates further so now we've got a water
leaving group, well what we can do, one way of thinking of it is it just leaves just
like a normal SN1 reaction but a way that makes a little more sense is
let's say that this pi bond goes ahead and puts that over there and so this is
essentially gonna kick off the water so basically we're losing H2O
right we've lost a water molecule, now that we have OR, this is
opening it up to again like this situation is analogous to this situation
here we've got a carbon doubly bound to an oxygen with a formal positive charge
carbon double bond oxygen, formal positive charge so this is susceptible to attack
again by another molecule of the alcohol so let's say this
R'OH, now we've got another R'OH, now let's say some other, once
again let's say it's another molecule of alcohol not sure what it is could be a
water molecule doesn't matter so I'm gonna grab this and go ahead neutralize
that and now we have R and we have a carbon right here attached to OR OR, and
that's what's called an acetal because we have a carbon to OR and OR, so
hemiacetal, hemi meaning almost like half, we've got OR OH is a hemiacetal
and an acetal is OR OR attached to the same carbon. so this is chemistry that
happens a lot in the body it has to do with how certain molecule cyclize
carbohydrates, things like that, and one interesting thing to note is that all of
these are acid-base equilibria and that means they're all completely
reversible so what this means is that to go in this in this reverse direction
this oxygen could protonate and we get this and then this oxygen could kick
that off and we get this and then water could attack right here
neutralize that and we get this we could lose a proton and we get this and then
and then this could protonate right there and we
get this guy and then this could come down and kick that off and we'd be
over here and then we could lose that proton and we'd be back to here, so
every single thing in this reaction is reversible so it's not that it's
unidirectional and we start here and we're going to get there but the thing
is that we can tweak this equilibrium and have one direction preferred for
example if we have an excess of alcohol if we have a lot of alcohol molecules in
the solution then every time an alcohol molecule sees something like this
gonna go ahead and attack, it's going to drive the direction this way that's
basic Le Chatelier's principle where we're adding one thing it's gonna
push the equilibrium to the other side. another thing we could do is remove
water if we remove water it's gonna push it towards this direction then again if
we add a lot of water and have not a lot of alcohol
molecules, and a lot of water molecules then water's gonna go ahead and attack here
push everything back in this direction so this is completely an acid-base
equilibrium we're seeing how the aldehydes with alcohols can generate
hemiacetals and acetals
so let's look at another functional group that forms very similarly to the
way hemiacetals and acetals form and that's an imine. so this time we're
gonna react this ketone instead of an alcohol were gonna react with this amine.
so let's take the lone pair of the nitrogen this is gonna be just fine to
go ahead and attack here and we're gonna get this over there and now when
we're in these kinds of aqueous conditions we can sort of take proton
transfer as rendered, because there's a lot of water molecules there's a lot of
molecules floating around in solution that are gonna be able to swap protons so let's
just say this oxyanion is gonna go ahead and grab two protons and then
sometimes when we just want to hand wave a proton transfer we can do these little
hook arrows so that is not a traditional electron pushing arrow, an electron pushing arrow
always goes from electron-rich to electron-poor but sometimes we just want to say
this proton is gone we don't care what took it, solvent molecule, whatever and so
that's gonna do that and we're going to neutralize that nitrogen so now we've
got this water leaving group and we've got this nitrogen over here, that has now
been neutralized, this can go ahead and kick off this water molecule
so here we are here once again, we have a positive charge and we've
formed this over here but instead of what we had with the alcohol where another
alcohol can attack over here instead we can just hook off this hydrogen again go
right there and we are going to form an imine, so an imine is anytime you
have a carbon with a double bond to a nitrogen so here's our imine right here
once again just like hemiacetal and acetal formation these are all equilibria
that are completely reversible so once again we could protonate right here we
could have water attack right here we could lose these protons, protonate
right there and then this could come down and then kick off the amine, so this
can all go all the way back that way as well but at any rate we are looking at
some acid-base equilibria, or some acidic conditions aqueous equilibria
that are going to be able to generate imines, as well as hemiacetals and acetals.
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