Chapter 5 Flashcards
allylic carbon
a carbon attached to a vinylic carbon
H2C=CHCH2-
vinylic carbon
a carbon that is attached by a double bond
H2C=CH-
alkene
hydrocarbons that contain a carbon-carbon double bond
alkene nomenclature
H2C=CH2 (ethene or ethylene)
CH3CH=CH2 (propene or propylene)
C-C double bonds
take precedence over alkyl groups and halogens in determining the main carbon chain
hydroxyl groups (OH)
have precedence over double bond
isomers of alkenes
-there is no rotation in C=C (double bonds) because of pi bonding
p orbitals
-carbon uses these orbitals in pi bonding
cis and trans
-in order to have cis and trans, carbons cannot have the same subgroups on one carbon
Z configuration
higher ranked substituents are on the same side (either top or bottom of c=c)
E configuration
higher ranked substituents are on opposite sides of the c=c
Z and E configuration
are determined by assigning priorities based on atomic number
Priority rules
Rule #1
higher atomic number takes precedence over lower
Priority rules
Rule #2
when two atoms directly attach to the same carbon of the double bond are identical, compare the atoms attached to these two on the basis of their atomic numbers. Precedence is determined at the first point of difference
Priority Rules
Rule #3
work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain
Priority rules #4
when working outward from the point of attachment, always evaluate substituent atoms one by one, never as a group
Priority rules #5
an atom that is multiply bonded to another atom is considered to be replicated as a substituent on that atom
increasing stability of alkenes
in general, alkenes with more highly substituted double bonds are more stable than isomers with less substituted double bonds
-van der waals strain plays are role for cis and trans isomers (trans more stable)
degrees of substitution
- monosubstituted: CH3CH2CH=CH2
- disubstituted: CH3CH=CHCH3
- trisubstitued: (CH3)2C=CHCH2CH3
- tetrasubstituted
alkyl groups
are better electron releasing substituents than hydrogen and are better able to stabilize an alkene
dehydration of alcohols
- the H and OH are lost from adjacent carbons
- acid catalyst is necessary
- converting alcohols to alkenes with H2SO4 (sulfuric acid)
which beta carbon does the H come of of in alcohol dehydration
the carbon with less hydrogens
-Zaitsev’s rule
beta elimination
- carbon with less hydrogens undergoes elimination
- most stable alkene
sterioselective reaction
a single starting material can yield two or more stereoisometric products
E1 and E2 mechanisms
- both reactions are promoted by acids
- the relative reactivity increases: primary<tertiary
E2 elimination
primary carbocation does not exist–>all steps happen at the same time
E1 elimination
secondary and tertiary alcohols
why do carbocations rearrange
tertiary carbocations are more stable than secondary carbocations
dehydrohalogenation
- the loss of a hydrogen and a halogen from an alkyl halide
- best method of making alkene
- uses a strong base
dehydrohalogentation of cycloalkyl halides
leads exclusively to cis cycloalkenes when the ring has fewer than 10 carbons
sodium ethoxide
- very strong base
- used in dehydrohalogenation
- made from Na + H-O-CH2CH3
rate of dehydrohalogenation
increasing rate: F<br></br><i>slowest rate</i>
I–>fastest rate
Dehydrohalogenation
both the alkyl halide and the base are involved in the rate determining step
E2 mechanism
has no carbocation formation; therefore, it is the better mechanism to form alkenes
dehydrohalogenation
in order for the H and X to be eliminated, they must be in the anti position
E2 mechanism of dehydrohalogenation of alkyl halides
key elements
- base-H bond making
- C-H bond breaking
- C=C pi bond formation
- C-halide breaking (weaker the bond, faster the reaction)
E1 mechanism of dehydrohalogenation
- base is missing
- alcohol is the solvent
- rate depends on what type of halide is there
- tertiary halide is the fastest
