5b Flashcards
alkenes are
nucleophilic
direct nucleophilic additions
attack the c of the carbonyl
conjugate addition
when the nucleophile attacks the double bond and its resonated up to the carbonyl o-
which then comes back down to the alkene which attacks a H+ to form an alkane.
for conjugate addition to take place,, what must be in conjuation
a cc double bond and a carbonyl
compounds that can undergo conjugate addition reactions are called
alpha, beta unsaturated carbonyl compounds.
can conjugated alkenes be electrophilic
yesss
bc they have a homo,, but also a lumo
how can we find the homo and lumo
count how many pi electrons there are (2 in each double bond)
draw lines going up
put 2 e- on each line until all the pi electrons are accounted for.
lowest line = no nodes
highest line = most number of nodes.
if it has a lumo,, that means it can accept e- into it.
when drawing the double bond in conjugation with the carbonyl,, we can tell which c the nuc would most likely attack, which one is it??
the c of the carbonyl = most positive,, this is the C that most nucs will attack.
due to good electrostatics,, only thing is that the orbital overlap isnt as good. bc its not a terminal carbon
when a double bond is in conjugation with a crabonyl,, we found that the c of the carbonyl is the most electrophilic,, what is the next electrophilic c,,
the terminal one.. aka the one that has a + charge when we resonate all the double bonds.
this has a lower + charge than the carbonyl C but has better orbital overlap as its terminal.
okay so based on the 2 diff types of +C we have,, one that has extensive orbital overlap and one that is favoured electrostatically,, which one is preferred by what
soft nucs prefer the terminal C,, the c that isnt as positive but dows provide extensive orbital overlap
hard nucs prefer the more electrostatically positive carbon that has less extensive orbital overlap.
what about the compound dictates if it will have direct or conjugate addition
the R group on the carbonyl.
a Cl or H will remove // keep the partical charge on carbonyl C extremely positive,, making hard nucs directly attack it
and R, OH or OR group will make the C of the carbonyl less partically positive,, making conjugate addition more likely.
hard nucs prefer + hard nuc examples
DIRECT ADDITION more positive carbon,, carbonyl carbon,, less extensive orbital overlap
F
OH
OR
H2O
NH3
ROH
RMgBr
Rli
soft nucs prefer what addition and what are examples of soft nucs
soft nucs prefer conjugate addition,, with the c that provides more extensive orbital overlap
I
Rs
alkenes
aromatic rings
middle nucs
CN
N3
RNH2
Br
okay describe conjugate addition
nuc attacks the end of a double bond and this is conjugated onto the o of the carbonyl.
what is the empty orbital for conjugate addition and how do we figure this out
depends on the number of double bonds // pi electrons u have
for a compound with 1 double bond and a carbonyl double bond. u have 4 pi electrons
so 1pi and 2pi will be filled as they can hokd 2e- each, which means that 3pi is empty,, and therefore the lumo
meaning 2pi is the homo.
so the lumo will be 3 pi .
see how many nodes there will be in each pi level (starting from 0 then 1, then 2 etc - this will help us when we draw the mo’s as we know when we have to switch the phases)
what is constructive overlap in conjugate addition between
the nuc sigma bond
the first p orbital ,, and the p orbital on the o (these 2 will have constructive overlap as the other p orbitals will be coloured and out of phase with them)
when we want to decide if smt will conjugately add or directy add,, what must we do
look at the molecule itself: is the R group next to te carbonyl gonna make the C’s partial charge highly + or less +
less positive = conjugate addition
more positive = direct addition
look at the nucleophile: hard nuc = mostly direct
weak nuc = mostly conjugate addition
what has more of a say tho: the moelcule or the nuc: in terms of if it will be a conjugate or direct addition
lowkey the molecule ngl.
strong nuc but not good moelcule = conjugate addition
not good moelcule meaning it makes the C of the carbon less partially positive.
