final exam Flashcards
double bond consists of
sigma bond and pi bond
triple bond consists of
2 pi bonds, 1 sigman bond
each carbon is sp hybridized with a linear geometry (180 degrees)
acyclic alkene structure
CnH2n
unsaturated hydrocarbons
alkenes and alkynes
have fewer than the max # of H atoms per carbon
cycloalkanes have the general formula:
CnH2n
alkynes have the formula:
CnH 2n-2
one degree of unsaturation:
each pi bond or ring removes two H atoms from a molecule
how to calculate degrees of saturation
compare actual number of H atoms in a compound to the max number of H atoms possible for the number of carbons present (if molecular were a straight chain alkane)
this gives total number of rings and/or pi bonds in a molecule
formulaic changes needed to be made to find degrees of unsaturation for heteroatoms
- for halogens: replace each ‘halogen’ with ‘H’
- for O or S: delete each oxy and sulfur
for nitrogen: for each ‘N’, delete 1 ‘H’ and 1 ‘N’
C2H4Cl2 =
C2H4H2 =
C2H6 =
CnH2n+2
NO degrees of unsaturated
substituted alkane
C5H6OCl2 =
C5H8O =
C5H8 =
CnH2n-2
TWO degrees of unsat; 2 pi bonds, 2 rings or (1 ring and 1 pi bond)
naming alkenes
1) find longest chain containing both carbon atoms of double bond
2) change -ane to -ene
3) number chain, use 1st number assigned to C=C
4) name and number substituents
E isomer has
two higher priority groups on OPP sides
Z isomer has
two higher priority groups on SAME side
what denotes priorities for substituents in an alkene?
E or Z
how to name an alkyne
1) change -ane to -yne
2) choose longest continuous chain that contains both atoms of the triple bond
3) number the chain to give the triple bond the LOWER number
diynes:
compounds w/ two triple bonds
triynes:
compounds w/ three triple bonds
compounds w/ both a double and triple bond is a
enyne
in alkynes, the first site of unsaturation is given
the lower number
two carbon alkyl group derived from acetylene is called an
ethynyl group
physical properties of alkenes and alkynes resemble
those of hydrocarbons of similar shape and molecular weight
alkenes and alkynes have
low melting pts and boiling points
melting and boiling pt increase number of carbons increases
alkenes and alkynes are not soluble in
water
alkenes and alkynes are soluble in
organic solvents
cis and trans isomeric alkenes often have
somewhat diff physical properties as consequence of diff dipoles
more polar isomers have
higher boiling points
cis alkene is more (x) than a trans alkene
POLAR
making bp slightly higher and more soluble in polar solvents
increasing number of double bonds =
lower melting point
symmetry effect:
cis bonds make these molecules less symmetrical and have poorer packing which affects melting pts
alkenes can be prepared from
alkyl halides and alcohols via elimination rxns
in said elimination rxns that are stereoselective/regioselective, most stable (x) is the
ALKENE
is the major product
alkynes are prepared by
eliminiation rxns
strong base removes 2 equivalents of HX from a vicinal/geminal dihalide = alkyne through successive e2 rxns
carbon atoms of a double bond are both
trigonal planar
elements of X and Y can be added to them from the same side/opp sides
syn addition takes place
when both X and Y are added from the SAME side
anti addition takes place
when X and Y are added from OPP sides
alkynes undergo
addition rxns b/c they contain relatively weak pi bonds like alkenes
in an addition rxns, two sequential rxns can take place:
addition of one equivalent of reagent forms an alkene
then add a second equivalent of reagent to = a product having 4 new bonds
alkenes’ oxidizing reactions
epoxidation, dihydroxylation, oxidative cleavage
alkynes oxidizing rxn
oxidative cleavage
hydrohalogenation is
the electrophilic addition of HX
to draw products of hydrohalogenation
1) locate C-C double bond
2) identify sigma bond of reagent that breaks H-X bond in hydrohalogenation
3) break pi bond of alkene and sigma bond of reagent
4) form two new sigma bonds to the C atoms of the double bond
mechanism of electrophilic addition consists of
two successive lewis acid-base rxns
step 1: alkene is the lewis base that donates an electron pair to H-BR (lewis acid)
step 2: Br- (lewis base) that donates an electron pair to the carbocation (lewis acid)
hydrohalogenation (markovnikov’s rule)
applies with an unsymmetrical alkene, HX can add to the double bond to = 2 constitutional isomers (only one is formed)
markovnikov’s rule states that
in the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom (carbon w/ greater number of H atoms)
basis of markovnikov’s rule is the
formation of a carbocation in the rate-determining step of the mechanism
in the addition of HX to an unsymmetrical alkene,
the H atom is added to the less substituted carbon to form the more stable/more substituted carbocation
trigonal planar atoms reacts w/ reagents from
two directions w/ equal probability
achiral starting materials yield
achiral products or new stereogenic centers are formed
halogenation (rxn stereochemistry)
slide 29
hydration (electrophilic addition of water) –>
addition of water to an alkene to form an alcohol
in a hydration rxn, alcohols add to alkenes –>
forming ethers by the same mechanism
i.e. addition of CH3OH to 2-methylpropene forms tert-butyl methyl ether (MTBE)
three consequences to the formation of carbocation intermediates
1) markovnikov’s rule holds
2) addition of H and OH occurs in both syn and anti fashion
3) carbocation rearrangements can occur
internal alkynes undergo
hydration w/ concentrated acid
terminal alkynes require presence of
HgSO4 to yield methyl ketones by addition of water
tautomers:
enol form
keto form
tautomers are
constitutional isomers that differ in the location of a double bond and H atom
two tautomers are in equilibrium w/ each other
tautomerization =
equilibrium w/ isomers
enol tautomer
has an OH group bonded to C=C
keto tautomer has
C=O and additional C-H bond
favored by equilibrium (C=O is much stronger than C=C)
hydroboration-oxidation is a
two step rxn sequence that converts an alkene into an alcohol
first step in hydroboration-oxidation
addition of elements (H and BH2 to pi bond of alkene) = intermediate alkylborane
proposed mechanism of hydroboration-oxidation involves
concerted addition of H and BH2 from same side of planar double bond
pi bond and h-bh2 bond are broken as two new sigma bonds are forms
trans state is four centered
with unsymmetrical alkenes,
boron atom bonds to the less substituted carbon atom
alkylboranes are oxidized without isolation w/
hydrogen peroxide
oxidation replaces the C-B bond w
C–O bond
forming new OH group w/ retention of configuration
syn addition of H and OH to a double bond =
anti Markovnikov
hydroboration-oxidation is a
two step rxn sequence that converts an alkyne to a carbonyl compound
addition of borane =
organoborane
oxidation with basic H2O2 =
an enol
tautomerization of the enol =
carbonyl compound
results of hydroboration-oxidation
addition of H2n to a 3x bond
hydroboration of an internal alkyne forms
a ketone
hydroboration of a terminal alkyne adds
BH2 to the less substituted, terminal carbon
after oxidation to the enol, tautomerization = aldehyde, carbonyl compounds
internal alkyne forms
ketone
terminal alkyne forms
aldehyde
sp hybridized C-H bonds are more acidic than
sp2 and sp3 hybridized C-H bonds
terminal alkynes are deprotanated w/ strong base
resulting ion = acetylide ion
acetylide anions react w/
unhindered alkyl halides = products of nucleophilic substitution
acetylides are
strong nucs
SN2 favored
fastest w/ CH3X and primary alkyl halides
steric hindrance around LG causes
2ndary and tertiary alkyl halides to undergo E2 mechanism
nuc substitution w/ acetylide anions form
new c-c bonds in high yield only w/ unhindered CH3X and primary alkyl halides
steric hindrance prevents
SN2 rxn
acetylide anions are
strong nucs that open epoxide rings by SN2
backside attacks occurs at the less
substituted end of the epioxide
markovnikov’s rule:
in addition of HX to an unsymmetrical alkene, the H atom bonds to the less substituted carbon
syn addition:
reagent is added from the same side
HYDROBORATION
anti addition
X and X are added from opp sides
HALOGENATION
halohydrin formation also involves
anti addition where X and OH are added from opp sides
syn and anti addition occur when
carbocations are formed
H and OH are added from same and opp sides
hydrohalogenation also involves both
syn and anti addition
H and X are added from the same and opp sides
ALL reactions of alkenes involve addition
weak pi bond is broken and two new sigma bonds are formed
calculate degrees of unsaturation
1) calculate max number of Hs = 2n +2
2) subtract actual # from max number and divide by 2
assign priorities in naming an alkene
E or Z (assign to substituents on each end of C=C)
E-isomer
two higher priority groups on opp sides
Z-isomer
two high priority groups on same side
when drawing products of an addition rxn, in an unsymmetrical alkene:
determine the regioselectivity
you can use reagents to identify the two groups added to
the C=C
carbocation intermediates gives
syn and anti products
concerted addition gives
syn products
converting an alkene to an alkyne
1) add halogen (X2) to an alkene
2) eliminate two equivalents of HX by treatment w/ strong base
draw product of an addition rxn
1) use reagents to ID the two groups added to C (3x bond) C
2) in an unsymmetrical alkyne, determine regioselectivity
3) draw products by breaking pi bonds and adding reagents
convert enol to keto tautomer in acid
1) locate C=C and H atom on OH group
2) add proton to C=C, draw two resonance structures
3) remove proton from OH group
convert keto tautomer to an enol in acid
1) locate C=o and H on alpha-carbon
2) add proton to C=O and draw two resonance structures
3) remove proton from alpha-carbon and draw C=C
comparing products of hydration of alkyne
1) use reagents to determine regioselectivity and add h20 to form enol
2) convert enol to its keto tautomer
comparing rxns of acetylide anions
1) classify alkyl halide
2) draw products
devise synthesis
1) compare carbon skeletons and functional groups
2) work backwards
3) work forwards
4) complete synthesis
