lecture 1 + 2 Flashcards
what hybridisation is seen in alkenes
sp2 hybridisation
1 sigma bond
1 pi bond
is the pi bond in alkenes the nucleophile or electrophile
nucleophile
if the the pi bond is the nuc,, what orbital are the e- in
the HOMO
what is Br2 in reactions
electrophile
where are e- accepted into in Br2 when it reacts
into the LUMO
sigma ** star
what happens to ethene when it reacts with Br2
1 Br attaches and forms a carbo cation intermediate
this is unstable as its primary
the lone pairs on Br donate e- density and it forms the BROMONIUM ION + Br-
product of ethene and Br2
BROMONIUM ION +
important intermediate
and Br-
describe the bromonium ion
2 carbons at the bottom linking to a Br at the top
the Br has a (+) charge
what can the BROMONIUM ion react with
can react with the Br-
how does the BROMONIUM ION react with Br-
the Br- attacks one of the bottom carbons
the e- in the C-Br breaks and goes into the Br+
forms 1-2 dibromoethane (dibromo derivative)
how else can we draw the reaction of ethene and Br2
• = attacks the Br-Br
• e- go to the end Br
• First Br’s lone pair attacks the = other end
what is hyperconjugation and how does it stabilise an alkene🌷
when e- in a CH sigma bond have a side on interaction with the empty C=C pi star orbital.
this leads to the 2e- in the CH sigma to be spread over the molecule + delocalised, increasing its stability
why can double bonds be nucleophiles
they’re electron rich
🌷a more substituted double bond is
more stable
also more reactive to electrophilic addition
why are more substituted double and triple bonds more reactive for electrophilic addition reactions
alkyl groups have an e- donating effect making the double / triple bond more e- rich and therefore more nucleophilic
why do F substituted double and triple bonds lose reactivity to electrophilic addition reactions
bc the F attracts some e-
double / triple bond is less nucleophilic
what reactions are used to make alkenes🌷
elimination reactions
waterfalls
halogenoalkanes are used
in electrophilic additions with halogenoalkanes, what C does the H and X bond to
H bonds to the C that’s bonded to the lowest amount of carbons
X bonds to the C that’s bonded to the highest amount of carbons
what is the meaning of a regioselective reaction🌷
there is a selective formation of one of the isomers
markovnikoffs product
H bonds to the C with little c
X bonds to the C with big c
makes a more stable CC+
what is the bonding interaction between an alkene and a HX🌷
the pi bonding HOMO (ALKENE)
the sigma star LUMO (HX)
in a gibbs energy of activation graph, where is the more stable CC+ intermediate
at a lower G value then the primary CC+
more stable = lower G energy value
hydride and alkyl shift🌷
when the H or CH3 move with 2 e- in order to form a more stable CC+
arrow from the CH bond to the CC+
an alkene and Br2 form
1,2-dibromoalkanes
what does 1,2-dibromo mean
that the Bromines are on adjacent carbons
antiaddition of Bromine to an alkene
makes an E isomer
first Br makes a ring
second Br ring opens (SN2) at any C opposite the first Br (LG) on the BROMONIUM ION +
another name for 1,2-dibromoalkanes
vicinal dibromides
the bromides are in the same vicinity (adjacent Cs)
syn addition of Br2 to an alkene
= is the nuc
Br2 is the electrophile
forms the bromonium ion
Br- attacks on the same C that the Br (LG) will remain attached to
what mix occurs when making the bromonium ion
racemic mix
the Br2 can attack both sides of the double bond
a racemic mix of bromonium ions leads to a racemic mix of
the dibromide product
(the alkene)
what occurs in a stereospecific reaction🌷
different stereoisomer reactants produce different stereoisomers of the product
the bromonium ions from a Z and E are
diastereoisomers
1 bromonium ion from a Z alkene gives🌷
2 possible products
SS or RR
enantiomers
one bromonium ion from a E alkene gives🌷
2 possible products
RS + SR (meso compounds)
bc the Br- attacking the Cs on the bromonium ion gives the same products
ACHIRAL - line of symmetry
the 2 products from alkene Z and 2 from alkene E are🌷
diastereoisomers
NOT ALL THE CHIRAL CENTRES CHANGE
Z = RR + SS
E = RS + SR
Z isomer and Br2 addition
alkene
bromonium ion
racemate
E isomer and Br2 addition
alkene
bromonium ion
meso compounds (achiral) SR RS
E and Z isomer and Br2 addition products
diastereomers
Br2 addition is
stereospecific
- product depends on reactant
alkene with Br2 and MeOH / H2O
SN2
alkene + Br -> bromonium ion
MeOH / H2O ring open
-H+ to give an XXX / alcohol
alkene + Br2 + MeOH/H2O
SN1
alkene + Br -> bromonium ion
ring open to form a CC+ (most stable)
MeOH / H2O attacks the CC+
-H+ to get XXX / alcohol products
why does the alkene + Br2 + MeOH only have 1 product when the alkene is asymmetric🌷
the MeOH attacks the most stable CC+
tertiary Carbon
ANTI ADDITION
when the Br electrophile and Br nucleophile attack at different sides
2nd Br must attack the C that’s opposite the Br (180* attack)
cyclohexene + Br2🌷
benzene attacks Br
bromonium ion forms (both dashed)
Br attacks the C at 180 to Br+
one Br is dashed, one is wedged (ANTI)
(enantiomers formed, racemic mix formed)
bromonium ion and a nuc attack description🌷
if the bromonium ion is symmetrical, the NUC can attack any C on the Bromonium ion
if asymmetric, we need to look at E isomers or the most stable CC+
electrophilic addition of HX to cyclohexene🌷
H is attacked by nuc (double bond)
no bromonium / ring system is made
CC+ is made
Br- then attacks this CC+
asymmetric alkene and HX electrophilic addition (arrow from double bond to HX)
follow markovnikoffs rule
H attached to C with little Cs
X attached to C with most Cs
phenyl group and an SH group is called a
thiol
how are thiols made🌷
benzene
H2S (nuc)
H2SO4 (electrophile / catalyst / H+ source)
what is an epoxide /oxirane🌷
a 3 member ring with O as one of the members
how do we form an epoxide from an alkene🌷
alkene + oxidation = epoxide
[O] reagent
what is a peroxy carboxylic acid🌷
a carboxylic acid with an additional ‘o’ between the o and c
it prevents resonance structures from forming when the H is removed, allowing the original O to be + and electrophilic (nucs attack it) 21.3 book
what is formed when a peroxy carboxylic acid is attacked by a nuc🌷
NUC OH and
R-C=O
|
O- stable carboxylate (good LG)
alkene + peroxy carboxylic acid reaction🌷
epoxidation - electrophilic addition
peroxy carboxylic acid donates an O to the C=C forming an epoxide + carboxylic acid
why is the peroxy carboxylic acid OH ‘O’ partially positive
the RCO2 withdraws electrons via induction / inductive effects
making the extra O more -
peroxy carboxylic acid reaction🌷
O from OH = + and is attacked
O-O e- break and form a double bond between O and C
old O and C double bond breaks to attack H from OH
arrow from H to the = of nuc
a trans alkene + peroxy carboxylic acid gives a🌷
trans epoxide
one R is dashed one R is wedged
a cis alkene and peroxy carboxylic acid makes a🌷
cis epoxide
both Rs from the alkene are dashed / wedged
meta chloro perbenzoic acid🌷
mCPBA
benzene with Cl meta and
peroxy carboxylic acid attached
benzene and mCPBA gives🌷
mcBA
cyclohexane epoxide
concerted process - all at the same time - no cationic / anionic intermediates
when mCPBA react with an alkene the product is🌷
EPOXIDE (cis or trans depending on the alkene)
mCBA
🌷diff between mCPBA and mCBA
mCPBA = meta chloro perbenzoic acid (3 Os in total) (peroxy carboxylic acid)
mCBA = product = meta chloro benzoic acid ( 2 Os only)( carboxylic acid with a benzene with Cl on meta position)
apart from epoxilation using an alkene and mCPBA, how else can an epoxide be formed🌷
alkene + DMDO -> acetone + EPOXIDE
what is dmdo
di methyl dioxirane
camping chair with two Os on the top
