Alkenes, Alkynes, and Elimination Rxns Flashcards
Higher melting point, cis or trans alkenes?
Trans
Higher boiling point, cis or trans alkenes?
Cis
What are the two types of elimination reactions (not E1 and E2)
Dehydrohalogenation and dehydration
What are the conditions required for dehydration?
Acid and high heat
What conditions favor E1?
Same as SN1: polar protic solvent, stability of the carbocation, highly substituted carbon, good leaving group, absence of good nucleophile.
How to favor E1 over SN1?
Higher temperatures favor E1
What types of products are favored in E2?
Bond will form on more highly substituted carbon and trans isomer will predominate.
How to favor E2 over SN2?
Steric hindrance is key, so highly substituted carbons and bulky bases make it harder for the SN2 to work. Also, a stronger base, because it will tend to tear off B-hydrogen before it reaches the a-carbon.
Alkene + (H2 + Pd/Pt/Ni) –> ?
Syn addition of H (reduction) to the double bond.
CH3CH=CH2 + HX –> ?
CH3-CXH-CH3 (Proceeds through most stable carbocation intermediate when the H+ adds to the less substituted carbon in the bond)
CH3CH=CHCH3 + Br2 –> ?
CH3CBrH-CBrHCH3 (Anti-addition proceeds through cyclic halonium ion intermediate. First double bond grabs one Br, leaving another Br-, forms a cyclic halonium, then Br- attacks attacks the ion on the opposite face.)
CH3-CH2OH + Acid + High heat –> ?
H2C=CH2 + H2O (Through E1 and E2 mechanisms. E2 if the OH is terminal since that would not form a stable cation.)
H2CCH=CH2 + Acid + H2O + low heat –> ?
H3C-CHOH-CH3 (Through carbocation intermediate).
H3CCH=CH2 + uv, peroxides, or oxygen + HBr –> ?
H3CCH2CH2Br + Br radical (Br free radical steals double bond and makes the secondary carbon a free radical [most stable], then the free radical attacks HBr and steals the H, leaving a Br free radical).
Alkene + BH3 + THF….. + H2O2 + OH- –> ?
Syn alcohol alkane
Alkene + cold, dilute KMnO4 –> ?
Syn vicinal diol (glycol) + MnO2(s)
Alkene + 1) KMnO4, OH-, heat and 2) H+ (acid wash) –> ?
Alkene cleaved at double bond and carbons oxidize as much as possible (single carbons become CO2, primary carbons become carboxylic acid, secondary carbons become ketones)
Alkene + 1) O3, CH2Cl2 2) Zn/H2O or (CH3)2S –> ?
Alkene cleaved at double bond and carbons oxidize to ketone or aldehyde.
Ozonolysis + mild reducing agent (NaBH4 or LiAlH4) –> ?
Aldehydes and ketones from ozonolysis reduced to alcohols
Alkene + CH3CO3H (peroxyacetic acid) or mCPBA (m-chloroperoxybenzoic acid) [these are both peroxycarboxylic acids] –> ?
Epoxide created between the carbons of the double bond.
Alkene + radical, heat, high pressure –> ?
Polymerization
Common name for ethyne
Acetylene
What’s a special property of terminal akynes?
The acidity of the terminal H, with pKa of 25
Two ways to synthesize alkynes?
1) Two rounds of elimination with geminal or vicinal dihalide with high heat and strong base.
2) Adding triple bond to existing haloalkane using strong base to pull of acidic hydrogen, which makes nucleophilic attack on the haloalkane.
Two ways to reduce alkynes to alkenes?
1) Lindlar’s catalyst: palladium on BaSO4 with quinoline. Produces cis isomer.
2) Na in liquid NH3 (<-33 C). Produces trans isomer and proceeds through free radical mechanism.
Propyne + Br2 –> ?
Trans dihaloalkene via cyclic halonium ion intermediate.
1-Butyne + HX + uv/peroxide –> ?
mostly trans 1-haloalkene
Explain hydroboration of alkyne
3 Alkynes + 1/2 B2H6… boron coordinates with three aklynes and followed by acetic acid wash makes 3 cis aklenes. If terminal alkynes, disubstituted borane used to prevent further boration of vinyl intermediate to alkane. Instead, vinylic borane intermediate oxidatively cleaved with H2O2 to make vinyl alcohol with tautomerizes to more stable aldehyde.
Alkyne + hot basic KMnO4 –> ?
Just like with alkenes, triple bonds are cleaved and carbons maximally oxidized.
Alkyne + ozone + H2O –> ?
Triple bond cleaved and carbons maximally oxidized
Explain addition of HX to an alkene
Electrons of double bond act as Lewis base and attack the H of HX creating a carbocation. The X- then attacks the carbocation to form a haloalkane.
Explain addition of X2 to alkene
A rapid process where the double bond acts as nucleophile and attacks X2, where X acts as the leaving group, forming a cyclic halonium ion intermediate. X- then attacks the opposite side for an anti-addition, forming trans dihaloalkane.
Explain addition of H2O to alkene
Performed under acidic conditions at low temperatures because at high temperatures the opposite reaction is favored. Double bond acts as nucleophile and attacks free H+ in solution, forming carbocation. Water attacks the carbocation and drops its H+ back into solution.
Explain the E1 reaction mechanism
A good leaving group leaves and forms a stable carbocation. A base attacks the hydrogen on neighboring carbon and the two electrons left from that bond move to form a double bond with the carbocation. This competes with the SN1 reaction, but is favored over it with high heat.
Explain the E2 reaction mechanism?
A strong, bulky base attacks the hydrogen of a carbon adjacent to a highly substituted carbon with a good leaving group under high temperatures. In one swoop, the base extracts the hydrogen, whose electrons move to make the double bond, and the leaving group leaves with the electrons from its bond. The trans isomer and the more heavily substituted carbon double bond will be favored.
