Alkene Simple alkenes are named much like alkanes, using the root name of the longest chain containing the double bond. The ending is changed from -ane to -ene. For example, “ethane” becomes “ethene,” “propane” becomes “propene,” and “cyclohexane” becomes “cyclohexene.”

In a cis-disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. In a trans-disubstituted alkene, the two dipole moments tend to cancel out. If an alkene is symmetrically trans disubstituted, the dipole moment is zero. For example, cis-but-2-ene has a nonzero dipole moment, but trans-but-2-ene has no measurable dipole moment.

Alkene Alkanes – saturated CnH2n Alkenes – unsaturated – not all bonds are saturated Nomenclature Common Names –ylene Ethylene CH2=CH2 Propylene CH2=CH-CH3 Butelene IUPAC Replace –ane ending with –ene Examples - Rules

To name alkenes, select the longest carbon chain which includes the carbons of the double bond. Remove the -ane suffix from the name of the alkane which corresponds to this chain. Add the suffix -ene. 2. Number this chain so that the first carbon of the double bond has the lowest number possible. 3-Methyl-3-hexene

C=C Double Bond Two bonds One -bond One -bond sp2 hybridization – planar shape Bond strength -bond 108 kcal/mol -bond 65 kcal/mol (poor overlap of -orbitals) Weak -bond accounts for functional group character (reactivity of alkenes)

Physical Properties of Alkenes Significant effect of molecular geometry of cis-alkenes on melting points [oC] Butane –138 trans-2-Butene –106 cis-2-Butene -139 Pentane –130 trans-2-Pentene –135 cis-2-pentene -180 Hexane –95 trans-2-Hexene –133 cis-2-Hexene –141 -bond weak electron-withdrawing character – slight polarity of Cs involved (imbalanced in cis-disubstituted alkenes)

Synthesis of Alkenes Dehydrohalogenation of alkyl halides dehydrohalogenation takes place in one step, in which a strong base abstracts a proton from one carbon atom as the leaving group leaves the adjacent carbon.

Formation of the Hofmann Product Bulky bases can also accomplish dehydrohalogenations that do not follow the Zaitsev rule. Steric hindrance often prevents a bulky base from abstracting the proton that leads to the most highly substituted alkene. In these cases, it abstracts a less hindered proton, often the one that leads to formation of the least highly substituted product, called the Hofmann product. The following reaction gives mostly the Zaitsev product with the relatively unhindered ethoxide ion, but mostly the Hofmann product with the bulky tert-butoxide ion.

2. Dehalogenation of vicinal dibromides Vicinal dibromides (two bromines on adjacent carbon atoms) are converted to alkenes by reduction with iodide ion in acetone.

3. Dehydration of alcohols Dehydration of alcohols is a common method for making alkenes. The word dehydration literally means “removal of water.” 4. Dehydrogenation of alkanes Dehydrogenation is the removal of H2 from a molecule

5-Reduction of alkynes

6-Wittig reaction

Synthesis of Alkenes KOH / EtOH C. H2SO4 1800C Zn / AcOH

Reactions of alkenes 1- Addition of hydrogen halides

Electrophilic Addition of HX Attack of electron-deficient atom at -bond H2C=CH2 + H-I  H2CH-CIH2 Mechanism via Carbocation Attack of H+ at -bond H2C +-C HH2 I- attacks carbocation Markovnikov Rule Proton will attach at less substituted carbon (form the more stable carbocation) H3CCH=CH2 + H-I  H3CCHI-CHH2 not formed: H3CCHH-CIH2 Carbocation stability: 3o> 2o > 1o > Methyl

Radical Additions – Anti-Markovnikov Products H3CCH=CH2 + H-I  H3CCHI-CHH2 Markovnikov Product H3CCH=CH2 + H-I  H3CCHH-CIH2 Anti-Markovnikov Product Caused by contamination of alkene with peroxyradicals (exposure to air) Mechanism via radicals

2- Acid-catalyzed hydration

Alcohol Synthesis – Electrophilic Hydration Follows Markovnikov rule Reverse of acid-catalyzed dehydration of alcohols (H3C) 2CH=CH2 + H-OH  (H3C) 2COH-CH2 (92% yield, Rx in 50% H2O:H2SO4) Attack of H+ at -bond RC +-C HR Nucleophilic attack of H2O at Carbocation Equilibrium: Dependent on Rx conditions Alcohol  conc. H2SO4 high temp.  Alkene Alcohol  dil. H2SO4, low temp.  Alkene

3-Hydroboration–oxidation

Hydroboration

4- Polymerization

5- Reduction: Catalytic Hydrogenation 6- Addition of Carbenes: Cyclopropanation

Oxidation of Alkenes with Peroxycarboxylic Acids Overall transformation :  C=C to epoxide Reagent : a peroxycarboxylic acid, RCO3H Regioselectivity : not relevant since both new bonds are the same, C-O- Stereoselectivity : syn since the two new C-O s bonds form at the same time from the peroxyacid.

7- Addition of halogens

Electrophilic Additions of Halogens to Alkenes Synthesis of vicinal halides H2C=CH2 + X2  H2CX-CXH2 Reaction requires no heat or UV light to initiate (as in Alkane halogenation) Reactivity F>> Cl> Br >>> I Reaction Mechanism via anti-addition Polarizable X-X bond leads to heterolytic cleavage - formation of cyclic Bromonium [(+)ive charge] intermediate at double bond

8-Halohydrin formation 9- Epoxidation

10- Ozonolysis

11- Potassium permanganate

Reactions of alkenes H2 / Pd X= Cl, Br, I X= Cl, Br or Ni CCl4 CCl4 O3 + Zn / H2O KMnO4 /OH or X2 / H2O