CHM 247 CH.7
Alkenes: Structure and Reactivity
• Alkene: A hydrocarbon that contains a carbon-carbon double bond, R C=CR2. 2
7.2 Calculating Degree of Unsaturation
• CnH 2nr an alkene versus C H n 2n+2
• Unsaturated: A molecule that has one or more multiple bonds. May also include rings.
• Organohalogen compounds (C, H, X, where X= F, Cl, Br, or I):Ahalogen substituent
acts as a replacement for hydrogen in an organic molecule. Therefore, when calculating
DOU, we add the number of halogens to the number of hydrogens.
• Organooxygen compounds (C, H, O): Oxygen forms 2 bonds, so it doesn’t affect the
formula of an equivalent hydrocarbon and can be ignored when calculating the DOU.
• Organinitrogen compounds (C, H, N): Nitrogen forms 3 bonds, so an organonitrogen
compound has one more hydrogen than a related hydrocarbon. Therefore, when
calculating DOU, we subtract the number of nitrogens from the number of hydrogens.
• Suffix is –ene.
1. Name the parent hydrocarbon.
2. Number the carbon atoms in the chain.
3. Write the full name. Use –diene, -triene, etc. for more than one double bond.
4. Recent naming change, but-2-ene not 2-butene.
• Common names used: Ethylene, not ethene. Propylene not propene. Isobutylene, not 2-
Methylpropene. Isoprene, not 2-Methyl-1,3-butadiene.
• Methylene group: A –CH - 2r =CH grou2.
• Vinyl group: A H 2=CH- substituent.
• Allyl group: A H 2=CHCH - su2stituent.
7.7 ElectrophilicAddition Reactions ofAlkenes
• Alkenes behave as nucleophiles (lewis bases) in polar reactions, donating a pair of
electrons from their electron rich C=C bond to an electrophile (lewis acid).
• Electrophilic addition reactions: The addition of an electrophile to a carbon-carbon
double bond to yield a saturated product.
• Electrophilic addition reaction mechanism example with HBr: 1)Ahydrogen atom on the
electrophile HBr is attacked by the π electrons from the nucleophilic double bond,
forming a new C-H bond on the less substituted Carbon. The intermediate molecule now
has a plus charge on the more substituted C, and a vacant p orbital. Simultaneously, 2
electrons from the H-Br bond move onto bromine, giving it a negative charge. 2)
Bromide ion donates an electron pair to the positively charged C atom, forming a C-Br
• Quick review:Anucleophilic atom is an electron-rich atom. Can be an atom bearing one
or more lone pairs but no formal positive charge, a formally neutral atom bearing a partial negative charge due to EN, or a carbon atom not bearing a formal positive charge and
multiple-bonded to another carbon atom.An electrophilic atom is electron deficient. Can
be an atom that lacks an octet of valence electrons in one or more resonance forms, or a
formally neutral atom bearing a partial positive charge but no lone pairs. Nucleophiles
• Electrophilic addition can occur using these reagents for example: HCl in ether, KI &
H PO , H O & H SO catalyst. (All Markovnikov).
3 4 2 2 4
7.8 Orientation of ElectrophilicAdditions: Markovnikov’s Rule
• Regiospecific: A term describing a reaction that occurs with a specific regiochemistry to
give a single product rather than a mixture of products.
• Markovnikov’s rule: A guide for determining the regiochemistry (orientation) of
electrophilic addition reactions. In the addition of HX to an alkene, the hydrogen atom
bonds to the alkene