Chapter 7 Day 23 Flashcards
Exam 3 (40 cards)
When can an alkyl halide undergo beta elimination (1, 2-elimination) to form an alkene?
When treated with a strong base
What are the main ideas of an E2 mechanism?
- Strong base used
- Loss of leaving group
- Formation of double bond
- Concerted (happens simultaneously)
- Bimolecular
Why can a substrate effect which mechanism is used?
-Tertiary substrate is too sterically hindered, preventing the nucleophile to penetrate and attack. Forces the nucleophile to act as a base and only E2 occurs
What isomer confirmation is generally more stable?
Trans formation because it creates less repulsion when the substituents are farther apart
How can differences in stability be quantified?
By comparing heats of combustion, for a reaction that has lower energy is more stable
How is the stability affected when more alkyl groups are added?
It becomes more a more stable alkene
What isomer confirmation is the only stable form when dealing with cyclic alkenes with less than 7 carbons?
The cis confirmation because the smaller the angles are, the more steric repulsion there is within the cyclic alkene. When the angles are larger, the substituents are farther apart, lowering steric repulsion.
What is Bredt’s rule?
The concept that we do not need to indicate if an alkene is cis or trans, we just assume that it is cis.
What are the two products formed when a substrate is able to have more than one beta carbon deprotonated by a strong base?
Zaitsev product & Hofmann product
What is the Zaitsev product?
- More stable alkene
- More substituted
- Major product
What is the Hofmann product?
- Less stable alkene
- Less substituted alkene
- Minor product
What does it mean for E2 to be regioselective?
When constitutional isomers are formed as the products of a reaction with one of them as a major product.
How can regioselectivity be controlled?
- By carefully choosing the strong base being used
- A bulky, sterically hindered use will favor the formation of the Hofmann product (minor product)
- An non bulky and unhindered base will favor the formation of the Zaitsev product (major product)
What does it mean for a base to be bulky?
A base that has lots of branching within the compound
Which isomer is commonly formed stereo-selectively in an E2 reaction?
Trans confirmation because it has a lower product energy which makes it more stable
In the transition state, the beta hydrogen and the leaving group of an E2 reaction be ____?
Co-planar
What is the difference between anti-coplanar and syn-coplanar?
- Anti-coplanar is when the groups are staggered and in the same plane
- Syn-coplanar is when the groups are eclipsed and in the same plane
Which rotamer is lower energy: anti-coplanar or sun-coplanar?
Anti-Coplanar, therefore this product will be formed since it is lower energy.
What is another word for anti-coplanar?
Anti-periplanar
Why would a less stable Z isomer be used over a more stable E isomer in an E2 reaction?
There is a requirement for an anti-planar transition state, and it will occur regardless if it is less stable.
What is there difference between a stereoselective reaction and a stereospecific reaction?
- Stereoselective reaction
- Gives multiple products, but one is favored
- Stereospecific reaction
- Gives one specific product
When is a stereoselective reaction occurring?
When the leaving group and the hydrogen are in trans formation
When is a stereospecific reaction occurring?
When the leaving group and the hydrogen are in cis formation
What is the rate formula for a bimolecular reaction?
Rate= k[substrate][nucleophile]
