04/30/2024 Flashcards
Halohydrins and Carbenes
What is halogenation and what are its features
it is the addition of a halogen (typically Br2 or Cl2) to an alkene.
It follows an anti addition and a cyclicbromonium ion formation
it has a similar mechanism to an SN2 reaction where the bromide ion attacks the cyclicbromonium ion from the back
What is a halohydrogenation and what are the features
it is an anti addition oh Br2 in an H20 solvent
rather than the bromide acting as nucleophile, the h20 is in abundance and acts as a nucleophile to add a h20 to the backside of the cyclicbromonium ion
in a hydrohalogenation, what is acting as a nucleophile
the h20 is in abundance and will preform the backside attack rather than the Br- anion
is a cyclicbromonium ion stable or unstable? why?
it is unstable because it is a 3 membered ring, it wants to open so it is reactive
what is a syn addition
it is addition to the same side
what is anti addition
it is an addition to opposite sides
where does the OH add to on a halohydrin reaction?
it adds to the more substituted carbon (most carbonation-like character)
what are some intermediates
carbocations, carbanions, radical, carbenes
why are reactive intermediates important
they make reactions happen
what are carbenes used for
to make cyclopropanes
why are cyclopropanes unstable
they have an angle strain (60 degrees deviation from 109.5)
torsional strain with eclipsing hydrogens
what is the charge of a carbene
it has no charge, it is neutral
what is a stereospecific reaction
it is the retention of stereochemistry
if your starting material is trans, your ending product is trans
if your starting material is cis, your ending material is cis
when we use :CH2 in a reaction, what forms and what is a stereochemical feature about it?
it forms a cyclopropane
it has stereoselectivity where the stereochemistry of the alkene is retained in the cyclopropane
what does it mean to be divalent and what is an example of it?
divalent means two bonds, carbenes are divalent, they have two bonds
how many valence electrons does a carbene have? why is this important
carbenes have 6 valence electrons
carbons want 8 electrons, so carbenes are highly reactive
what is special about carbenes? do they act as nucleophiles or electrophiles
they can act as both, they are deficient in electrons (6) and can receive electrons, which gives it an electrophilic character
but it can also donate electrons due to the lone pair on the carbon, this gives it a nucleophilic character
explain how alkenes and carbenes interact
since carbenes are electron deficient, they receive electron density from the electron rich region of the pi bond
since carbenes have a lone pair, they can donate electrons to the pi bond and act as a nucleophile
this forms two bonds at the same time, which is why stereospecificity occurs
why are carbenes stereospecific
they donate and accept electrons in a concerted mechanism, which gives no opportunity for free rotation and retains stereochemistry
will you ever get a mix of cis and trans when using carbenes
no
how can we use carbenes? what needs to happen first?
carbenes need to be generated “in situ” and then must be reacted with something else quickly
what does it mean to be generated “In-Situ”
it means it is made during the reaction
how is diazomethane used to make carbenes
diazomethane when activated with heat or light breaks its C-N bond to form a carbene “in situ” and nitrogen gas (favorable and stable)
if you use hydroxide and CHCl3 on an alkene what will form
a cyclopropane with a geminal dihalide
what is the function of OH and CHBr3, CHI3, CHCl3?
bromoform, iodoform, and chloroform are used to create a carbene that can create a cyclopropane with a gem-dihalide
what is the effect of the three chlorides on the carbon in chloroform?
they pull electron density away from the carbon, which makes the C-H more easily broken
what is the pka of chloroform
24
what is the process of using OH/CHCl3 with an alkene
first the OH deprotonates CHCl3 in an alpha elimination
Then the Chloride leaves and forms a carbene :CCl2
what is a simmons smith reaction
it creates a cyclopropane through a carbenoid
what is a carbenoid
a carbenoid is something that is “Carbon-like” where it acts like a carbene but doesn’t actually form one
its good to use when we don’t want gas or dihalides, it is generally safer
What is involved in Simmons Smith
CH2I2 + Zn(Cu)
draw a spirocyclic compound
its in your notes
