ALKENES AND ALKYNES

2ND SHIFTING - ORGCHEM LEC

Unsaturated hydrocarbon

hydrocarbon (HC) molecule in which one or more

carbon– carbon multiple bonds (double bonds, triple bonds, or both) are present.

Alkenes

has one or more C—C double bonds.

Alkynes

has one or more C—C triple bonds.

Aromatic HC

has a special type of “delocalized” bonding.

Alkenes and Alkynes

follows similar reaction mechanism

Aromatic HC

follows different reaction mechanism from Alkenes and Alkynes

Index of Hydrogen Deficiency (IHD)

• “number of sites of unsaturation”

• number of pairs of hydrogen atoms that must be removed from

  the corresponding “saturated” formula to produce the molecular

  formula of the compound of interest.

• result from cyclic structures and the presence

  of multiple bonds (double/triple bonds).

• computed from compounds with C, N, H, O, S, and X.

Classic Formula of IHD

Unified Formula of IHD

CnH2n

General Formula for Alkene

CnH2n-2

General Formula for Cycloalkene

2, 4

Alkenes have __ fewer -H atoms and cycloalkenes have _ fewer -H

atoms compared to alkanes with the same # of C’s.

-alkenyl

Alkene as substituent is called

Alkenes

commonly found in pheromones (sex attractant) and terpenes (aliphatic volatile oil)

gas

C2-C4 alkenes

liquid

C5-C17 alkenes with 1 C=C

solid at RT

more than C17 alkenes

CnH2n-2

General Formula for Alkyne

CnH2n-4

General Formula for Cycloalkyne

4

Alkynes have __ fewer hydrogens compared to alkanes with the same #

of C’s; occurs less commonly compared to alkenes.

closely similar; 2 pi bonds

The reactions of alkynes are ____ to that of the alkenes and

focuses on the carbons participating in the triple bond.

However, alkynes can accommodate one more addition reaction

because it possess _____ in the functional group.

polar mechanism; electrophilic addition reaction

Alkenes/Alkynes follows a ____ on its reaction, specifically

an ______.

less; more

The pi electrons are moved to the ___ stable

carbon and the ___ stable carbon

becomes an electrophile.

1

Only the more stable carbon in the double

bond will be an electrophile, and only __

reaction product will be formed.

Regiospecific reaction

• when only ONE of the two addition product orientation

  if formed.

• follows the Markonikov’s Rule.

Markonikov’s Rule

• proposed by Vladimir Markonikov after observing the addition of HX to an alkene.

• “that in the addition reaction of alkenes, the more highly

  substituted carbocation is formed rather than the less highly substitute

  one”.

• more stable carbocation is formed over the

  less stable one.

more stable

When a pi bond is cleaved, the _____

carbon becomes a carbocation (C+).

adds

In the presence of other reagents that can serve as

nucleophile (Nü), the nucleophilic portion of the

reagent ___ to the carbocation and forms a new bond.

Hydration

Electrophilic Addition Reactions (AE) of Alkenes:

• addition of water (H2O) to alkenes

• destructive to molecules

H3PO4, HCl, H2SO4, and H+

Catalyst of Hydration

nucleophile (-OH); 2° ROH product

The _____ from H2O is added to the

carbocation and forms a ____.

hydroboration

Hydration reaction follows Markonikov’s Rule, except ____.

acid

STEPS IN HYDRATION:

1. pi-bond reacts with the (a=?) catalyst, providing the

   H+. This result into bond cleavage and formation of

   carbocation intermediate.

2. H2O acts as a nucleophile and adds itself to the

   carbocation electrophile. This forms a (b=?).

3. The electrons between H and OH in the protonated

   alcohol will (c=?) back to the O atom.

   This will release the proton, regenerating the H+ taken

   from the acid catalyst and forming the final product, a

   (d=?).

a=

protonated alcohol

STEPS IN HYDRATION:

1. pi-bond reacts with the (a=?) catalyst, providing the

   H+. This result into bond cleavage and formation of

   carbocation intermediate.

2. H2O acts as a nucleophile and adds itself to the

   carbocation electrophile. This forms a (b=?).

3. The electrons between H and OH in the protonated

   alcohol will (c=?) back to the O atom.

   This will release the proton, regenerating the H+ taken

   from the acid catalyst and forming the final product, a

   (d=?).

b=

delocalize

STEPS IN HYDRATION:

1. pi-bond reacts with the (a=?) catalyst, providing the

   H+. This result into bond cleavage and formation of

   carbocation intermediate.

2. H2O acts as a nucleophile and adds itself to the

   carbocation electrophile. This forms a (b=?).

3. The electrons between H and OH in the protonated

   alcohol will (c=?) back to the O atom.

   This will release the proton, regenerating the H+ taken

   from the acid catalyst and forming the final product, a

   (d=?).

c=

2◦ alcohol

STEPS IN HYDRATION:

1. pi-bond reacts with the (a=?) catalyst, providing the

   H+. This result into bond cleavage and formation of

   carbocation intermediate.

2. H2O acts as a nucleophile and adds itself to the

   carbocation electrophile. This forms a (b=?).

3. The electrons between H and OH in the protonated

   alcohol will (c=?) back to the O atom.

   This will release the proton, regenerating the H+ taken

   from the acid catalyst and forming the final product, a

   (d=?).

d=

1. BH3 / THF

2. H2O2, NaOH, H2O

Catalyst of Hydroboration

1. Hg(OAc)2, H2O

2. NaBH4

Catalyst of Oxymercuration-Demercuration

Fumarase

Catalyst of Enzymatic Hydration

Ether Formation

Electrophilic Addition Reactions (AE) of Alkenes:

• addition of alcohol (ROH) to alkene

• follows Markonikov’s rule to determine the more stable

  carbocation to be formed.

H+ (acid)

Catalyst of Ether Formation

(RO-)

Ether Formation:

The nucleophile __ from ROH is added to the more stable

carbocation (C+).

H

STEPS IN ETHER FORMATION:

The double bond (C=C) electrons abstracts the (a=?) from the acid catalyst or even from the RO—H itself, breaking the double bond and producing a carbocation.

Subsequently, the carbocation is then attacked by the nucleophile (b=?) group attaching itself to the stable carbocation and forming an (c=?) product.

a=

(RO-)

STEPS IN ETHER FORMATION:

The double bond (C=C) electrons abstracts the (a=?) from the acid catalyst or even from the RO—H itself, breaking the double bond and producing a carbocation.

Subsequently, the carbocation is then attacked by the nucleophile (b=?) group attaching itself to the stable carbocation and forming an (c=?) product.

b=

ether

STEPS IN ETHER FORMATION:

The double bond (C=C) electrons abstracts the (a=?) from the acid catalyst or even from the RO—H itself, breaking the double bond and producing a carbocation.

Subsequently, the carbocation is then attacked by the nucleophile (b=?) group attaching itself to the stable carbocation and forming an (c=?) product.

c=

Hydrohalogenation

Electrophilic Addition Reactions (AE) of Alkenes:

• addition of hydrogen halide (HX) to alkenes.

• follows Markonikov’s Rule, where the nucleophilic (X-) is added to the more stable carbocation in an ether catalyzed solution.

monohalogenated alkane

Product of Hydrohalogenation

ether

Catalyst of Hydrohalogenation

nucleophile (X-) from hydrogen halides (HF, HCl, HBr, or HI)

added to alkenes to form monohalogenated alkane as a product in Hydrohalogenation

Halogenation

Electrophilic Addition Reactions (AE) of Alkenes:

• follows the same mechanism as HX and H2O addition, with the

  exception of Br2 addition occurring with anti-stereochemistry (trans-).

nucleophile (X-) from single mole of halogen (Cl2 or Br2)

added to alkenes in Halogenation

DCM/CCl4 catalyst

catalyst of halogenation

dihalogenated alkanes

product of halogenation

anti-stereochemistry (trans-)

orientation of halogenation

Halohydrin Formation

Electrophilic Addition Reactions (AE) of Alkenes:

• addition of halogen (X2) in H2O to alkenes

• follow Markonikov’s rule

nucleophile of -X and -OH

added to alkene in halohydrin formation

halohydrin product

product of halohydrin formation

anti-stereochemistry (trans-)

orientation of halohydrin formation

Hydrogenation/Reduction

Electrophilic Addition Reactions (AE) of Alkenes:

• addition of hydrogen (H2) to alkenes.

• double bond is reduced to a single bond.

H2 atom

added to alkenes in Hydrogenation

Pd/PtO2

catalyst of Hydrogenation

saturated products

product of Hydrogenation

syn stereochemistry (cis-).

orientation of hydrogenation

Epoxidation

Electrophilic Addition Reactions (AE) of Alkenes:

• addition of peracid (RCOOOH) to alkene

Peracid/Peroxy Acid

• class of organic compound with a -OOH group.

• (RCOOOH)

m-chloroperoxybenzoic acid (mCPBA)

compound classified as a

peroxy acid and a common reagent used for epoxidation

epoxide → vicinal diol

product of epoxidation

Oxidation

Electrophilic Addition Reactions (AE) of Alkenes:

• exposure of alkene to an oxidizing agent (e.g., KMnO4)

Hydroxylation

mild oxidation

KMnO4 in basic solution (H2O/NaOH)

catalyst of Hydroxylation

vicinal diol

product of Hydroxylation

syn stereochemistry (cis-)

orientation of Hydroxylation

Bond cleavage

strong oxidation

oxidizing agent (KMnO4) in acidic solution (H3O+).

catalyst of Bond Cleavage

ketone

2 oxidized bonds = ?

RCOOH

3 oxidized bonds = ?

CO2

4 oxidized bonds = ?

Ozonolysis

Electrophilic Addition Reactions (AE) of Alkenes:

• oxidation reaction of alkene using ozone (O3).

• oxidative cleavage of the C-C multiple bonds (double/triple

  bonds) among alkenes/alkynes.

O3 with a metal catalysts Zn

catalyst of Ozonolysis

carbonyl products with the more stable carbocation

product of Ozonolysis

Polymerization

Electrophilic Addition Reactions (AE) of Alkenes:

• addition of a radical to alkenes.

• radical adds up to the alkene to undergo propagation of the

  polymer.

• The newly formed radical repeats the radicalization

  process as many times to elongate the polymer.

Benzyloxy radical (BzO•).

added to alkene in Polymerization

longer carbon chains with repeating polymers

product of Polymerization

Hydrogenation

Electrophilic Addition Reactions (AE) of Alkynes:

• addition of hydrogen (H2) to alkynes.

Pd/C or Lindlar’s Catalyst

catalysts of Hydrogenation in Alkynes

H2

added to alkynes in Hydrogenation

syn stereochemistry (cis-).

orientation of Hydrogenation in Lindlar’s catalyst in Alkynes

Hydrohalogenation

Electrophilic Addition Reactions (AE) of Alkynes:

• addition of hydrogen halide (HX) to alkynes.

• follows Markonikov’s Rule, where the nucleophilic (X-) is added to the more stable carbocation formed.

HX

added to alkynes in Hydrohalogenation

monohalogenated alkenes.

product of Hydrohalogenation in Alkynes

geminal halides or dihalide alkanes

HYDROHALOGENATION:

With excess HX, alkynes can undergo further reaction forming _____.

Halogenation

Electrophilic Addition Reactions (AE) of Alkynes:

• addition of halides (X2) to alkynes.

• follows the same mechanism with X2 and alkenes

X2

added to alkynes in Halogenation

vicinal halide

product of Halogenation in Alkynes

tetrasubstituted alkyl halide

HALOGENATION:

With excess X2, the reaction can result into the formation of

_______.

anti-stereochemistry (trans-) product

orientation of Halogenation in Alkynes

Hydration

Electrophilic Addition Reactions (AE) of Alkynes:

• addition of water (H2O) to alkynes

H2O

added to Alkynes in Hydration

H2SO4/HgSO4

catalyst of Hydration in Alkynes

enol (alkene alcohol)

intermediate product of Hydration in Alkynes