Chemistry 3719 Chapter 5
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What is the hybridization pattern of carbon atoms in a C=C double bond?
Why is cis/trans isomerism possible in alkenes but not acyclic alkanes?
Rotation around the double bond is not possible, therefore the substituents attached are stuck in their particular orientation (stereochemistry). Single bonds, e.g. in acyclic alkanes, rotate freely
Which are held more tightly, pi electrons or sigma electrons?
Sigma electrons; they are found along the internuclear axis
What type of intermediate is formed in a unimolecular elimination reaction?
A carbocation (rate-determining step is the loss of a leaving group)
How many steps are involved in a typical E2 process?
One step; the reaction is concerted
What is Zaitsev's rule?
Typically in elimination reactions to form alkenes the more highly substituted alkene is favoured
Why is a tetra-substituted alkene more stable than a di-substituted isomer?
Alkyl groups help stabilize the sp2 hybrid C atoms in the double bond by pushing electron density towards those carbons (i.e. a positive inductive effect). The more alkyl groups attached the better.
Give the name of the major product formed when 1-methylcyclohexanol undergoes acid-catalyzed elimination.
Give an acceptable name for CH3CH2CH2CH2CH=CH2.
Rank Cl, Br, I, and F in their ability to act as leaving groups in E2 reactions (best = 1, worst = 4)
I = 1; Br = 2; Cl = 3; F = 4
Why don't simple alcohols generally undergo base-promoted elimination (i.e. E2) reactions?
The base will go after the OH proton instead (pKa ~ 16).
In the lab, 4-methylcyclohexanol undergoes acid-promoted elimination to give a small amount of 1-methylcyclohexene. How does this happen?
Through sequential rearrangement of the secondary carbocation that is formed to give a good tertiary carbocation, which then loses a proton to give that product.
Does tetrachloroethene have a dipole or not?
No, the electron-withdrawing effects of the 4 Cl atoms will cancel each other out
Which is more stable, trans-2-butene or cis-2-butene and why?
Trans-2-butene will be more stable as the two large CH3 groups will be on opposite sides of the double bond and therefore not interacting unfavourably.
Why do HBr and HCl give substitution products with alcohols while H2SO4 and H3PO4 give elimination products?
HBr and HCl give the nucleophilic Br- and Cl- species which induce substitution; H2SO4 and H3PO4 give resonance-stabilized (and therefore weakly nucleophilic) conjugate bases which allow E2 to occur.
Why is trans-cycloheptene only stable at very low temperatures?
The molecule is extremely strained; making a trans-cycloalkene gets easier as the ring size increases.
What is the name of the major organic product formed when 2-methyl-2-propanol undergoes E1 dehydration with sulfuric acid?
Why is an anti alignment of proton and leaving group preferred in E2 reactions over a gauche alignment?
The orbitals required for the double bond are already aligned properly in the anti situation
Are simple alkenes generally soluble in water or not?
No, they are non-polar and incompatible with highly polar water.
What is the general shape and hybridization of a carbocation formed in E1 reactions?
Trigonal planar and sp2 hybridized
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