Ch. 8 – Alkyl Halides & Elimination Reactions Flashcards
What is an elimination reaction?
loss of starting materials to form a pi bond
What substance do alkyl halides undergo elimination reactions with?
bronsted-lowry bases
What is dehydrohalogenation?
removal of H and X; beta elimination because it involves the loss of elements from two adjacent atoms
What is the alpha carbon?
carbon bonded to X
What is the beta carbon and can there be more than one?
carbon bonded to H right next to alpha carbon; yes
What three steps occur for general elimination mechanism?
1) base removes proton from beta carbon and forms H-B+
2) electron pair in beta C-H forms new pi bond between alpha and beta
3) new pi bond boots off electron pair in C-X bond which goes with leaving group
What are the four most common dehydrohalogenation bases?
(alkoxides) Na+/ -OH sodium hydroxide K+ / -OH potassium hydroxide Na + / -OCH3 sodium methoxide Na+ / -OCH2Ch3 sodium ethoxide K+ / -O(CH3)3 potassium tert-butoxide
T/F: Having more than one beta carbon will lead to multiple products that are constitutional isomers.
true
What are alkenes?
products of elimination, C=C
What are important factors about alkenes (geom, hybridization)?
sp2 hybridized, trigonal planar (120º)
What does double bond consist of? What types of overlap cause these bond types?
one sigma and one pi bond
- sigma caused by end-end of sp2 overlap (lies in plane)
- pi caused by side-side of 2p overlap (lies perpendicular and has restricted rotation
What are mono/di/tri/tetrasubstituted alkenes?
number of carbon atoms bonded to carbons of double bonds
Does a double bond have free or restricted rotation? What does this cause?
restricted rotation, stereoisomers (cis and trans)
When are diastereomers possible?
whenever two groups on each end of C=C double bond are different
Are cis or trans alkenes more stable?
trans because groups are further apart and there are reduced steric interactions
How does the number of R groups bonded to C=C affect stability?
more R groups = more stable
Why do R groups increase stability?
they are sp3 hybridized and donate electron density to lower % s character sp2 carbon (inductive effect)
Do sp2 or sp3 more readily accept electron density?
sp2
What are the two types of elimination mechanisms?
E2 and E1
What is an E2 mechanism?
one step (concerted), second order, bimolecular
What is rate equation for E2 mech?
rate = k[R-X][base]
What are general steps for E2 mechanism?
1) base removes proton from beta carbon
2) C-H bond breaks and electron pair forms pi bond
3) leaving group breaks off and takes second electron pair
What are the characteristics of the energy diagram for E2 mech?
- one barrier (hump)
- transition state has 4 partial bonds
- entropy favors products (more molecules of product)
- products lower energy
What factors affect rate of E2 mech?
alkyl halide: more substituted = faster rate (opposite of Sn2)
strong base = faster rxn
better leaving group = faster rxn
polar aprotic solvent = faster rxn
T/F: When there are two or more beta carbons, equal amounts of product are made.
false, one product predominates
What is the more major product in elimination reactions?
major product = more stable product (more subs on double bond)
What is the Zaitsev rule?
major product in beta elimination has more substituted double bond
What does regioselective mean?
when a reaction yields more of one constitutional isomer (E2 is regioselective)
Why does Zaitsev rule occur?
double bond is partially formed in transition state so increasing stability of double bond lowers Es for reaction and rate increases
What product is major when stereoisomers are formed?
more stable stereoisomer (trans)
What does stereoselective mean?
when reaction produces more of one stereoisomer when two or more are possible
What is E1 mechanim?
two steps, first order, unimolecular
What is rate equation for E1 mechanism?
rate = k[alkyl halide]
Which bond breaks first in E1 mech?
bond to leaving group breaks before pi bond is formed
What are steps of E1 mech?
1) heterolysis of C-X bond forms carbocation (RDS)
2) base removes proton from carbon adjacent to carbocation and electron pair in C-H bon forms new pi bond
What are characteristics of energy diagram for E1?
- two humps
- step one is higher hump (RDS)
How do elimination reactions produce alkynes?
two consecutive elimination reactions form two pi bons
What are the two starting materials that can be used to form an alkyne?
vicinal dihalide or geminal dihalide
What is a vicinal dihalide?
has two X atoms on adjacent carbon atoms
What is a geminal dihalide?
has two X atoms on same carbon atoms
What is the intermediate product formed when forming alkynes?
vinyl halide
What type of bases are needed to form an alkyne?
VERY strong bases (usually amides -NH2) because it is harder to remove H and X from sp2 hybridized carbons
T/F: Sn1 and E1 have exact same first step
true, both produce carbocation
How do Sn1 and E1 differ in step two?
Sn1: nuc attacks carbocation to form substitution product
E1: base removes a proton and forms pi bond
T/F: the conditions for E1 to occur and the same as for Sn1 to occur
True (3º alkyl halide, weak nucleophile/base, polar protic solvent)
What happens when E1 or Sn1 reaction could occur?
forms mixture
How many atoms does the transition state of E2 reaction have in its plane?
4 (co planar C-x and C-H)
What are the two conformations for the E2 transition state?
syn periplanar and anti periplanar
What is syn and anti periplanar?
syn periplanar is when H and X are on the same side
anti periplanar is when H and X are on opposite sides
What is needed to E2 reaction, syn or anti periplanar geometry?
anti periplanar because it is staggered and therefore lower in energy which allows electron rich base and leaving group to be further away from eachother
For halocycloalkanes, how must geometry occur for E2 reaction to take place?
must be anti periplanar and have trans diaxial position for X and H
What is trans diaxial geometry?
C-X and C-H are both in anxial position of ring
Are both of the cis conformation rings reactive for E2? How many difference products can cis form?
no, only X axial conformation is reactive
can form two different products because it has two beta hydrogens in axial position (minor or major depending on alkene substitution)
Are both trans ring conformations reactive for E2? How many different products can trans ring form?
only trans with axial halogen is reactive
can only form one product because there is only one axial beta hydrogen
Should stereochemistry be drawn for alkene products?
alkenes and subs should be drawn in plane because they are trigonal planar
What is the most important factor in determining E1 or E2?
strength of base; E2 = strong base; E1 = weak base
Can E1 reactions occur for methyl or 1º?
No, but E2 can
What is the only reaction that methyl can undergo?
Sn2
What reactions can 1º undergo?
Sn2, E2
What reactions can 2º undergo?
Sn2, E2, Sn1, E1
What reactions can 3º undergo?
E2, Sn1, E1
Do 1º benzylic or allylic compounds undergo E2?
no, only Sn2 or Sn1 (or E1 if there is resonance)
Do 3º benzylic or allylic compounds undergo Sn2?
no, only E2, Sn1 and E1
When do E1 reactions go for 1º allylic and benzylic ompounds?
not for benzylic but if there is resonance in allylic
