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Chapter 7

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Barb Morra

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. 7.3 NamingAlkenes • Suffix is –ene. • Steps: 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 bond. • 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 attack electrophiles. • 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
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