Alkenes E The double bond consists of a bond and a bond bond
from head-on overlap of sp 2 orbitals bond from side-on overlap of
p orbitals bond bond Rotating a double bond requires breaking the
bond NO FREE ROTATION at room temperature Contain at least one C=C
double bond General formula: C n H 2n (like cycloalkanes) Each
carbon atom in a C=C double bond is sp 2 hybridized A bond is
stronger than a bond. bonds bond
Slide 3
Alkene Nomenclature (Naming) 1.Parent chain = longest chain
that includes the double bond(s) 2.The double bonds have priority
and must have the lowest number(s) possible 3.The first C atom in
the C=C bond indicates the double bonds location (or number in
naming) 4.Name, number, & alphabetize substituents as usual
5.Replace ane ending with ene ending Two double bonds: -diene;
three double bonds: -triene 6.Put double bond number in front of
entire root name (i.e. 2-pentene indicates the double bond starts
on carbon 2) 7.Cyclic alkenes: number the atoms in the ring
starting with the double bond 2-hexene 3-butyl-2,4-hexadiene
Slide 4
Naming Practice 4-ethyl-3,5-dimethyl-2-heptene 2,4-hexadiene
3,4-dimethyl-1,3-pentadiene 4-isopropyl-3,5-dimethyl-
1,3,5-heptatriene
Slide 5
Cis-trans isomerism in alkenes Substituents will stay on the
same or opposite sides of the double bond (no C=C bond rotation) X
2-butene (same side) 2-butene (opposite sides) For cis-trans
isomerism, each C in the double bond must have 2 different
substituents attached (i.e. a C and a H, etc.) Determining cis or
trans: follow the parent chain through the double bond
cis-3-methyl-2-heptenecis trans- cis-
Slide 6
Cis/Trans Naming Practice trans-4-ethyl-3,5-dimethyl- 2-heptene
trans,trans-2,4-hexadiene 3,4-dimethyl-1,3-pentadiene
trans,trans-4-isopropyl-3,5- dimethyl-1,3,5-heptatriene No Cis or
Trans Both Hs Both CH 3 s No Cis or Trans Both Hs
Slide 7
Important Common Names Vinyl (branch) Propylene Allyl (branch)
Ethylene R R CH 2 HC i.e. polypropylene = milk jugs i.e. polyvinyl
chloride = PVC pipe i.e. diallyllysergamide = derivative of LSD
i.e. polyethylene = plastic bags
Slide 8
Arranging many double bonds Cumulated C=C double bonds all in a
row: C=C=C=C Conjugated Single and double bonds alternate:
C=CC=CC=C Isolated >1 single bond between double bonds:
C=CCC=CCC=C Lycopene 11 conjugated double bonds 2 isolated double
bonds
Slide 9
Reactions of Alkenes Alkanes substitution reactions R-H + A-B
R-A + H-B R = residue, a generic alkyl group Alkenes addition
reactions + A-B Thermodynamics: H rxn = bonds broken bonds formed =
( bond + bond) ( bond + bond) Exothermic reaction bond is
electron-rich
Slide 10
Alkene Addition Reactions + A-B Reaction Hydrohalogenation
(addition of H-X, X = halogen) Hydration (addition of H 2 O)
Addition of halogens Hydrogenation (addition of H 2 ) A-B H-F,
H-Cl, H-Br, H-I H-OH Br-Br, Cl-Cl, F-F H-H
Slide 11
Reactions at the Double Bond bond is electron-rich Acts as a
nucleophile Attracts electrophiles to form bonds Electrophile:
electron-lover Electron deficient reagent (often + charged cations)
seeks e - in reactions Nucleophile: nucleus-lover electron-rich
reagent (often charged anions) tries to donate e - to an
electrophile in reactions, forming bonds
Slide 12
Markovnikovs Rule Consider the reaction Vladimir Markovnikov
Two products are possible Experimentally, only 2-chloropropane is
formed + 1-propene 1-chloropropane2-chloropropane Markovnikovs
Rule: The alkene carbon with the most H atoms gets the H
Hydrohalogenation (H-X), hydration (H 2 O) of alkenes Why? Look at
the reaction mechanism to find out... C C
Slide 13
Carbocation Stability Relative carbocation stability: 3 > 2
>> 1 > methyl >>>> 231methyl Most stableLeast
stable Why? Electrons can drift into the empty p orbital from C-H
bonds on neighboring C atoms to help stabilize Result: The final
product comes from the most stable carbocation intermediate
Slide 14
Mechanism of Hydrohalogenation Step 1: Alkene electrons attack
H +, forming a carbocation Step 2: Cl - nucleophile forms bond with
carbocation or+ carbocation The carbocation formed in step 1
determines the final product
Slide 15
Hydrohalogenation Predict the product of the following
hydrohalogenation reaction + HBr OR Markovnikov Product
Anti-Markovnikov Product Remember: Markovnikovs rule says that the
H (from HBr) will bond to the alkene C with the most Hs
Slide 16
Remember: Markovnikovs rule says that the H (from H 2 O) will
bond to the alkene C with the most Hs Hydration Predict the product
of the following hydration reaction + H 2 O OR Markovnikov Product
Anti-Markovnikov Product
Slide 17
Mechanism of Hydration (R + H 2 O) Step 1: bond attacks
electrophilic H + catalyst forming carbocation Step 2: Nucleophilic
H 2 O attacks carbocation, forming oxonium ion Step 3: Oxonium ion
loses a proton, regenerating H + catalyst Most stable carbocation +
Markovnikov product oxonium ion
Slide 18
Mechanism of Halogen Addition (R + X 2 ) Step 1: Br 2 is
polarized by electrons, Br + attaches to alkene Step 2:Br + blocks
access to one face of the alkene Br - adds to the other face (anti
addition) bromonium ion intermediate + trans isomer is the only
product
Slide 19
Halogenation + Br 2 The Brs will add to opposite sides of a
RING (anti addition) The Brs will be forced into a trans
conformation ALWAYS Trans isomer
Slide 20
Mechanism of Hydrogenation (R + H 2 ) Occurs in the presence of
a metal catalyst (like Pt) Pt HH + Pt H H Step 1: H 2 adsorbs to
catalyst surface Step 2: Both H atoms add to same face of alkene
(syn addition) Pt H H cis isomer is the only product
Slide 21
Hydrogenation Occurs in the presence of a metal catalyst (like
Pt) + H 2 /Pt Both Hs will add to the same side of a RING (syn
addition) If branches are present, they will be forced into a cis
conformation cis isomer
Slide 22
Hydrogenation of alkenes vegetable oils + H 2 /Pt unsaturated
saturated Contains double bonds Contains NO double bonds
Slide 23
Hydrohalogenation Determine any reactant(s) that could yield
the given product of the following hydrohalogenation reaction + HBr
trans-3-methyl-2-hexene 3-methyl-1-hexene + HBr Major product using
Ms rule Only possible reactant for this product
Slide 24
Hydrohalogenation Determine any reactant(s) that could yield
the given product of the following hydrohalogenation reaction + HBr
trans-3-methyl-2-hexene trans-3-methyl-3-hexene All three reactants
could give this product 2-ethyl-1-pentene
Slide 25
Halogenation Determine any reactant(s) that could yield the
given product of the following halogenation reaction
trans-3,5-dimethyl-2-heptene Only one possible reactant in this
case + Br 2
Slide 26
Alkene Reaction Summary Hydrohalogenation (+ HX) Hydration (+ H
2 O) Halogenation (+ X 2 ) Hydrogenation (H 2 /Pt) Markovnikovs
rule Cis/trans with rings
Slide 27
Polymerization of Alkenes Polymer: a large molecule made by
linking together small repeat units called monomers Polymerization
mechanism: radical chain reaction MonomerPolymer ethene (ethylene)
polyethylene propene (propylene) polypropylene
Slide 28
E Alkynes Contain at least one C C triple bond with
sp-hybridized C atoms Triple bond: one bond (sp orbitals), two
bonds (p orbitals) Naming: triple bond indicated by yne ending
Reactivity: same addition reactions as alkenes Use 2 equivalents of
addition reagent (i.e. + 2HCl) Use Markovnikovs rule in the same
manner ethyne (acetylene) 4-methyl-1-pentyne
Slide 29
Alkyne Naming Practice 3,4-dimethyl-1-pentyne 2-methyl-3-hexyne
3-methyl-1-pentyne 2,5-dimethyl-3-hexyne
Slide 30
Hydrohalogenation of Alkynes Predict the product of the
following hydrohalogenation reaction + 2HBr ? Break the reaction
into two steps, adding 1 HBr each time to the multiple bond + HBr
Final Product 1-pentyne Markovnikovs rule still applies
Slide 31
Hydration of Alkynes Predict the product of the following
hydration reaction + 2H 2 O ? Break the reaction into two steps,
adding H 2 O each time to the multiple bond + H 2 O Final Product
3,3-dimethyl-1-butyne Markovnikovs rule still applies
Slide 32
Halogenation of Alkynes Predict the product of the following
halogenation reaction + 2Br 2 ? Break the reaction into two steps,
adding Br 2 each time to the multiple bond + Br 2 Final Product
3,3-dimethyl-1-butyne
Slide 33
Hydrogenation of Alkynes Predict the product of the following
hydrogenation reaction + 2H 2 ? This reaction will simply turn the
alkyne to an alkene, and then to an alkane + H 2 Final Product
3,3-dimethyl-1-butyne
Slide 34
Alkyne Reaction Practice 1. 2. 3. 4. 5. + 2HCl + 2H 2 O + 2Br 2
+ 2H 2 + 2HCl AND