Alkene - Properties, Nomenclature, Reaction, Isomers, Synthesis

Properties: Similar to alkane; non-polar, flammable

 

Nomenclature:

 

C=C; cumulative -C=C=C-

 

• Polyunsaturated fatty acid: 2 or more C=C

 

• Allene: adjacent C=C=C

 

• Vinyl group: H2C=CH-

 

• Methylene group: H2C=

 

• Allyl group: H2C=CH-CH2-

 

• Vinyl halide: halide replaces -H on >C=C<

 

• Conjugated: alternate C-C and C=C (resonance)

 

• Alkadiene, 2 conjugated C=C; example: butadiene;

s-cis and s-trans (rotate about C-C bond)

 

• Alkatriene, 3 conjugated C=C

 

• Annulene: conjugated monocyclic compound;

example: [6] annulene = benzene

 

• Aromatic cyclic ions: cyclopentadiene anion,

cycloheptatriene cation (6 electrons)

 

• Add -ene to prefix; Use # to denote C=C position

 

• Isolated C-C=C-C-

 

Isomers:

no free rotation of C=C

 

• E/Z; prioritize groups by atomic weight (Z - higher

priority groups on the same side)

 

• For noncyclic: cis is less stable (steric hindrance).

For cyclic, cis more stable.

 

• Hofmann Rule: Form the least-substituted alkene

 

• Markovnikof Addition: H adds to C with most -H’s

 

• Zaitsev Elimination: Form alkene with more

 

 

Synthesis:

 

• Dehydrate alcohol (H+, heat) (elimination)

 

• Dehydrohalogenate haloalkane (base, heat)

 

• Dehalogenate vic dihalide (Zn, acetic acid)

 

• Hydrogenate alkyne:

 

• syn, Z/cis-isomer (H2,P-2 catalyst)

 

• anti, E/trans-isomer (Li, NH3, -78 ̊C)

 

• Wittig, aldehyde/ketone + phosphorous ylide

 

Reaction:

 

• Combustion (O2)

 

• Hydrate to 2 ̊/3 ̊ alcohol (H+, H2O); 1 ̊ from ethene;

can rearrange (Markovnikov)

 

• Hydrate to alcohol; hydroborate/oxidize

(THF/B2H6,H2O2/OH-) (syn, anti-Markovnikov)

 

• Oxymercurate-demercurate to alcohol

 

• Hydrohalogenate (HX) (Markovnikov)

 

• Halogenate (Br2/Cl2), vic dihaloalkane (X2,CCl4; anti)

• Halohydrin (X2, H2O; anti-addition)

 

• Hydroxylate to form a 1,2-diol (KMnO4, cold OH-syn addition)

• Oxidize to carboxylic acid (KMnO4, hot OH-)

 

• Ozonolyze to ketone (O3; Zn, H2O)

 

• Hydrogenate to alkane (Pt, H2; syn-addition)

 

• Free radical polymerization

 

• Alkadiene Reaction

 

• allylic halogenation (Cl2, heat)

 

• Diels-Alder: cycloalkene from diene + alkene/alkyne

Post By : Pankaj 25 Dec, 2018 1831 views Chemistry