Chapter 7 Flashcards
alkenes are hydrocarbons with _______ bonded carbons. This is also called _______
double
olefin
Sigma bonds around the double bonded C’s are _______ hybridized
sp2
Unhybridized p orbitals with ___ electron each will overlap, forming the _______ bond. This overlap requires the 2 ends of the molecule to be coplanar
1
pi
Sp2 hybrid orbitals have _______ s character than sp3 hybrid orbitals. This results in _______ bonds
more
shorter (S is closer than P orbital; 2 bonds draw atoms closer)
Rotation around double bonded C is _______ without breaking the pi bond. Therefore, Cis and Trans isomers _______ be interconverted
impossible
cannot
Alkanes are called saturated hydrocarbons because they contain only _______ bonds
single (sigma)
alkenes are _______ because they are able to add Hs in the presence of a catalyst
unsaturated
unsaturation values are _______
double bond: _______
Ring: _______
triple bond: _______
additive
1
1
2
What is the formula for calculating elements of unsaturation?
((2n+2) - #H - #X + #N) / 2
n= carbons
H= hydrogens
X= halogens
N= Nitrogen
O and S have no effect
Nomenclature of alkenes
1) longest carbon chain that contains _______
2) Lowest # to _______
3) When numbering is equal for double bonds, look at _______ then use _______
4) If double bonds are positioned in C1 and C2 of a ring, assume they are _______
5) finally, add geometric isomerism prefix:
1) double bonds
2) double bonds
3) substituents, alphabetization
4) cis
5) cis/trans and E/Z
What are the 4 double bond containing substituents?
1) methylene
2) vinyl
3) allyl
4) isopropenyl
Cis trans isomerism is not possible if one of the double bonded C’s has _______ groups
identical
You only use E/Z nomenclature when you have ___ or ____ different substituents
3 (trisubstituted)
4 (tetrasubstituted)
Use Cahn-Ingold Prelog rules when assigning priority for _______ nomenclature
E/Z
When naming alkenes, E/Z are placed in _______ at the front of the name. It is also accompanied by a __ indicating the position of the alkene in the molecule
parentheses
#
In E/Z nomenclature __ is where high priority are going to be the same __ is where high priority are across
Z
E
The physical properties of alkenes are _______ to those of alkanes
- _______ dense than water
- branched alkenes have _______ bp
- slightly _______
- Cis alkenes have ________ bp than trans alkenes
similar
- less
- lower
- polar
- higher
The chemical properties of alkenes are _______ than those of alkanes
- more reactive because of the _______ bond electrons
different
pi
Combustion of an alkene and hydrogenation of an alkene can provide valuable data to the _______of the double-bonded C
Stability
More substituted double bonds are usually _______ stable
more
_______: alkenes with more highly substituted double bonds are usually more stable
zaitsev’s rules
trans isomers are generally _______ stable than the corresponding cis isomer
more
Alkenes with more alkyl groups are more stable because the alkyl groups are _______, which help to stabalize the pi-system (double bond)
EDG’s
Rings that are __-membered or larger can easily accommodate double bonds and these cycloalkenes react much like straight chain alkanes
5
Cis isomer is _______ stable than trans in small cycloalkanes
more
Rings must have at least __ carbons to form a stable trans double bond. The larger the ring the more stable the trans will be
8
Bredt’s Rule: a bridged bicyclic compound can’t have a double bond at a bridgehead position unless one of the rings contains at least __ carbon atoms
8
Conjugated systems require __ or more p-orbitals in a row which leads to enhanced stability of the system
3
All elimination reactions produce _______ bonds. The H+ is grabbed from the C that is adjacent to the ________.
double
LG
Elimination reactions are also known as _______ reactions
dehydrohalogenation
E1 will always be a minor product of Sn1 reactions, which will reduce the % yield of a reaction. This minor product will be ignored except for a very specialized case: dehydration of _______ with no _______ available
dehydration of alcohols with no nucleophile available
All E2 reactions are _______
- do they have any intermediates?
concerted
no intermediates
What is the rate law for E2 reactions?
rate = k [R-LG] [base]
What is the order of reactivity of alkyl halides for E2 reactions?
3° > 2° > 1°
- reflects stablility of double bonds
Most E2 reactions follow _______: the more highly substituted alkene is the major product. What are the 2 exceptions?
Zaitsev’s rules
1) Hofmann’s rule
2) Conjugation
Hofmann’s rule: when a base is _______ the _______ highly substituted alkene is the major product
big and bulky
less
What are the 4 big bases that follow Hofmann’s rule?
1) tert-butoxide (KOTBu)
2) LDA (has negative charge)
3) Et3N
4) 2,6-dimethylpyridine
Conjugation: if the possibility of forming a conjugated product exists, it will _______ be the major product
always
trumps Hofmann’s rule and Zaitsev’s rule
The E2 reaction follows a concerted mechanism, which requires a specific geometric arrangement so that the orbitals of the bonds _______ (LG) can overlap with those being _______ (double bond)
broken
formed
What are the 2 coplanar arrangments for the reactions of E2 reactions?
1) anti-coplanar (low E)
2) syn-coplanar (high E)
Which coplanar arrangement is favored in most E2 reactions?
anti-coplanar
syn-coplanar reactions are only favored in _______ systems
restricted (rings)
anti-coplanar elimination: E2 is a _______ reaction
stereospecific
(wedges on top, dashes on bottom)
For E2 reactions to work in cyclohexane rings, the H and the LG must be _______ to form the double bond.
coplanar
- Only axial H can be used when LG is also axial
2 axial bonds are trans to each other are called _______
trans-diaxial
If there are no trans-diaxial H and LG then _______ occurs
no reaction
