Chapter 7 Flashcards
alkene = olefins meaning?
oil forming gas
what is the functional group of an alkene?
the carbon carbon double bond
What gives an alkene its reactivity?
the carbon carbon double bond
saturated
no carbon double bonds
unsaturated
carbon double bond, decreases the number of hydrogens in the molecule by 2
what are elements of unsaturation
pi bonds and rings
Degree of Unsaturation / Index of Hydrogen deficiency equation
2C+2+N-H-X / 2
how much is a double bond and ring in a DoU?
- double bond = 1
- ring = 1
when naming alkenes, how should you number the chain?
so that the double bond has the lowest possible number, then signify where the double bond is
(ex: either 2-pentene or pent-2-ene)
in a ring, where is the double bond assumed to be by its naming?
between the first and second carbon
how do you name an alkene with multiple double bonds?
use di-, tri-, and tetra- before the ending of -ene
Z = cis or trans?
cis
E = cis or trans?
trans
when should you use cis/ trans?
simple, disubstituted alkenes
when should you use E/Z?
tetra and tri substituted alkenes
in cyclic compounds, is cis or trans more stable?
cis
how many carbons does a ring need to have in order for trans to be stable?
8 carbons
all cycloalkenes are assumed to be _________ unless otherwise specifically named _________
cis, trans
which has a greater dipole, cis or trans?
cis
which has a higher boiling point, cis or trans?
cis
define hydrogenation
an alkene transforming into an alkane
the _______ substituted the double bond, the lower its heat of hydrogenation
MORE
because is is more stable
more substituted = more _____________
stable
what causes ring strain in cycloalkenes?
- small ring
- trans double bond
Bredt’s Rule
bridged, bicyclic compound cannot have a double bond at a bridgehead position UNLESS one of the rings contains at least 8 carbon atoms
physical properties of alkenes
- low boiling points (increaded by high molecular weight and suface area)
- less dense than water
- nonpolar
what increases an alkenes boiling point?
- increased mass
- decrease branching
what is an elimination reaction?
they produce double bonded structures and a leaving group (that is often a halide)
E2 Reaction: strong or weak nucleophile?
strong
E2 Reaction: unsubstitutued or highly substituted leaving group?
highly substituted (want bulky)
E2 Reaction: concerted to two-step?
concerted
E2 Reaction: rate
- bimolecular
- k[Nu][RX]
E2 Reaction: stereochemistry
anti-coplanar
E2 Reaction: reactivity
3>2>1>Me
E1 Reaction: strong or weak?
weak
E1 Reaction: unsubstitutued or highly substituted leaving group?
highly substituted
E1 Reaction: concerted or two step?
two step
E1 Reaction: rate
- unimolecular
- k[RX]
E1 Reaction: stereochemistry
doesn’t matter
Elimination steps
- leaving group leaves
- solvent takes a proton and forms a double bond
E1 rate limiting step
transition state
Zaitsev’s Rule
if more than one elimination product possible, the most substituted alkene is the major product
E2: Hofmann
Hofmann reaction
bulky bases cannot form the most stable because they cannot fit in the smaller spots, this will lead to a less stable product
which is more stable, Zaitsev or Hofmann?
Zaitsev
what does the stereochemistry need to be in order for an elimination reaction to occur?
LG and H need to be 180 degrees from each other (anti-coplanar) –> find the beta hydrogens
examples of strong alkyl halides
EtO- , -OH , RSH, ZRS
examples of weak alkyl halides
EtOH, H2O
dehydration of alcohols produces what?
an alkene and H3O+
dehydration of alcohols steps
- O will take an H
- H2O will leave and a positive will be created
- a H will be removed to form a double bond
which reactions favor a strong base?
SN2 or E2
which reactions favor a weak base?
SN1 or E1
which reaction favors primary halides?
SN2
which reaction favors tertiary halides?
SN1, E1, E2 (CANNOT OCCUR IN SN2)
which reaction favors heat?
E1 or E2 (unless it is primary, then SN2)
which reaction does bulky bases favor?
E1 or E2
common bulky bases (makes E2)
- t-buO-
- LDA
- LiTMP
common bases (makes E2)
- RO-
- RC (triple bond) C
- NH2-
- H-
- heat
the more R groups on the alcohol, the ______ to dehydrate
easier (3>2>1)
the specific elimination mechanism depends on how easily the molecule will form a ______________
carbocation (3 and 2 make good carbocations)
