elect_addn_alkene Flashcards
Define nucleophile.
A nucleophile is an electron-rich molecule/ion with a lone pair of electrons that is attracted to a positively charged region of a different molecule/ion.
Define electrophile.
An electrophile is an electron-poor molecule/ion (or region of a molecule/ion) which attracts an electron-rich (or negatively-charged) molecule/ion.
Why is a primary carbocation more stable than a methyl carbocation?
A methyl carbocation has only hydrogens attached to a positive charge. A primary carbocation has one alkyl group (methyl, ethyl etc.) attached to a carbocation. A primary carbocation is more stable as the result of induction (electrons pulled along the sigma bond towards the positive charge) and most importantly hyperconjugation.
What orbitals are involved in hyperconjugation?
Hyperconjugation is the result of overlap of electrons in a C-C or C-H sigma molecular orbital and a C 2p atomic orbital.
Describe hydrohalogenation in terms of regiochemistry and stereochemistry.
Hydrohalogenation occurs via the most stable carbocation intermediate. Therefore, the halide is added Markovnikov. Again because of the trigonal planar carbocation intermediate, two stereoisomers will form (most often) in equal proportions.
Under what conditions will a hydride or alkyl shift occur?
A hydride or alkyl shift will occur when such a shift results in a more stable carbocation (e.g. secondary to tertiary)
What functional group is formed as a result of an acid catalyzed addition of water to an alkene?
An alcohol functional group is formed.
What functional group is formed as a result of an acid catalyzed addition of an alcohol to an alkene?
An ether functional group is formed.
Describe acid catalyzed addition of water (or alcohol) in terms of regiochemistry and stereochemistry.
Acid catalyzed addtion of water (or alcohol) occurs via the most stable carbocation intermediate. Therefore, the water (or alcohol) is added Markovnikov. Again because of the trigonal planar carbocation intermediate, two stereoisomers will form (most often) in equal proportions.
Explain why halogenation occurs via anti-addition.
Halogenation (such as bromonation or chloronation) occurs anti because the reaction involves the formation of a bromonium (or chloronium) ion intermediate. Such an intermediate requires that the nucleophilic attack occur from the opposite side of the bromo (or chloro) group. Therefore the two bromo groups (or two choro groups) will add on opposite sides of what was the double bond. This has definite consequences for the resulting stereochemistry. Further this outcome is more clearly seen when the original alkene is part of a ring structure. The result are bromines (or chlorines) on opposite sides of the ring (trans vs cis).
What solvent is used to avoid solvent additions in a halogenation reaction?
To avoid solvent additions in a halogenation reaction a nonpolar, non-reactive solvent like tetrachloromethane (CCl4) is used.
What solvents, when used in a halogenation reaction, will result in a solvent addition?
When water or an alcohol are used as solvents in a halogenation reaction, the solvents are added through an attack of the bromonium (or chloronium) ion intermediate in an anti-addition. The product of such a reaction is refered to as a halohydrin.
Describe the conditions necessary for a syn anti-Markovnikov addition of an OH group to an alkene.
A syn anti-Markovnikov requires two steps: 1) BH3,THF; 2) H2O2, NaOH, H2O
Describe the regio- and stereo- chemistry of a catalytic hydrogenation reaction
Catalytic hydrogenation has no regiochemistry because a hydrogen is added to one of each of the carbons in the alkene. The stereochemistry is the result of a syn addition.
What concept is defined by the following statement?
An electron-rich molecule/ion with a lone pair of electrons that is attracted to a positively charged region of a different molecule/ion.
Nucleophile
