04/30/2024 Flashcards

Halohydrins and Carbenes

1
Q

What is halogenation and what are its features

A

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

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2
Q

What is a halohydrogenation and what are the features

A

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

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3
Q

in a hydrohalogenation, what is acting as a nucleophile

A

the h20 is in abundance and will preform the backside attack rather than the Br- anion

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4
Q

is a cyclicbromonium ion stable or unstable? why?

A

it is unstable because it is a 3 membered ring, it wants to open so it is reactive

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5
Q

what is a syn addition

A

it is addition to the same side

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6
Q

what is anti addition

A

it is an addition to opposite sides

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7
Q

where does the OH add to on a halohydrin reaction?

A

it adds to the more substituted carbon (most carbonation-like character)

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8
Q

what are some intermediates

A

carbocations, carbanions, radical, carbenes

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9
Q

why are reactive intermediates important

A

they make reactions happen

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10
Q

what are carbenes used for

A

to make cyclopropanes

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11
Q

why are cyclopropanes unstable

A

they have an angle strain (60 degrees deviation from 109.5)

torsional strain with eclipsing hydrogens

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12
Q

what is the charge of a carbene

A

it has no charge, it is neutral

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13
Q

what is a stereospecific reaction

A

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

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14
Q

when we use :CH2 in a reaction, what forms and what is a stereochemical feature about it?

A

it forms a cyclopropane

it has stereoselectivity where the stereochemistry of the alkene is retained in the cyclopropane

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15
Q

what does it mean to be divalent and what is an example of it?

A

divalent means two bonds, carbenes are divalent, they have two bonds

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16
Q

how many valence electrons does a carbene have? why is this important

A

carbenes have 6 valence electrons

carbons want 8 electrons, so carbenes are highly reactive

17
Q

what is special about carbenes? do they act as nucleophiles or electrophiles

A

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

18
Q

explain how alkenes and carbenes interact

A

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

19
Q

why are carbenes stereospecific

A

they donate and accept electrons in a concerted mechanism, which gives no opportunity for free rotation and retains stereochemistry

20
Q

will you ever get a mix of cis and trans when using carbenes

A

no

21
Q

how can we use carbenes? what needs to happen first?

A

carbenes need to be generated “in situ” and then must be reacted with something else quickly

22
Q

what does it mean to be generated “In-Situ”

A

it means it is made during the reaction

23
Q

how is diazomethane used to make carbenes

A

diazomethane when activated with heat or light breaks its C-N bond to form a carbene “in situ” and nitrogen gas (favorable and stable)

24
Q

if you use hydroxide and CHCl3 on an alkene what will form

A

a cyclopropane with a geminal dihalide

25
Q

what is the function of OH and CHBr3, CHI3, CHCl3?

A

bromoform, iodoform, and chloroform are used to create a carbene that can create a cyclopropane with a gem-dihalide

26
Q

what is the effect of the three chlorides on the carbon in chloroform?

A

they pull electron density away from the carbon, which makes the C-H more easily broken

27
Q

what is the pka of chloroform

A

24

28
Q

what is the process of using OH/CHCl3 with an alkene

A

first the OH deprotonates CHCl3 in an alpha elimination
Then the Chloride leaves and forms a carbene :CCl2

29
Q

what is a simmons smith reaction

A

it creates a cyclopropane through a carbenoid

30
Q

what is a carbenoid

A

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

31
Q

What is involved in Simmons Smith

A

CH2I2 + Zn(Cu)

32
Q

draw a spirocyclic compound

A

its in your notes