Orgo1 Chp 9 - Alkyne reactions

Question 0

NaNH2, NH3 and H2O to geminal or vicinal dihalide alkanes

Answer 0

Requires 3 equivalences of NaNH2, NH3 to create an alkane by removal of the H and halide groups. The first and second equivalence removes one H and one halide each, the third equivalence neutralizes the acidic product by replacing the remaining H with Na then the H2O replaces the Na with H, these last two reactions occur back and forth to prevent the acidic product from consuming the rest of the NaNH2 base.

Question 1

NaNH2, NH3

Answer 1

Strong Base used in SN1 Substitution of H for Na, also can be used for SN2 alkylation of terminal alkynes with a alkyl halide.
In alkylation of terminal alkyne, first Na is substituted in place of the terminal H, then the alkyl halide replaces the Na for the alkyl group.
- effective only with Primary alkyl Halides. *Secondary or tertiary alkyl halides undergo elimination in the presence of a strong base.

Question 2

Alkyne + 2H2 + catalyst (Pt, Pd, Ni, Rh)

Answer 2

Hydrogenation of Alkyne
creates an alkane by the addition of 4 H’s over the triple bond, alkene is an intermediate. CAN NOT STOP AT ALKENE– SEPARATE RXN FOR THIS.

Question 3

Alkyne + H2 + Lindlar’s Catalyst (Pd/CaCO3)

Answer 3

Hydrogenation of alkyne to create ALKENE. Lindlar’s is a “poisoned” catalyst that will stop after the first addition of 2 H’s over the double bond creating only an alkene.
* PRODUCES ONLY Z ALKENES!!!! (diff. rxn to produce E)

Question 4

Alkyne + Na/NH3 (or Li/NH3 or K/NH3)

Answer 4

Hydrogenation of alkyne to produce ALKENE. Stops after the first addition of H’s over the triple bond = an alkene. Similar to Lindlar’s except: PRODUCES E ALKENES!

Question 5

Alkyne + hydrohalide (HBr, HCl, HF)

Answer 5

Hydrohalogenation of Alkyne
adds H and halide across the triple bond.
* Obey’s Markovnikov’s rule! (H adds to least substituted side to = stable carbocation, halide adds to most highly substituted side)
2 Equivalents of HX = alkane
HF will work!

Question 6

Alkyne + HBr + peroxide

Answer 6

Free-Radical addition of HBr across an alkyne

same mechanism as alkenes. Adds H and Br across triple bond opposite to Markonikov’s rule.

Question 7

Alkyne + H2SO4 + HgSO4 + H2O

Answer 7

Hydration of alkynes
Would expect the addition of OH and H across the alkyne to produce triple bond + alcohol. This however does not occur. Results in alkane plus ketone. Enol (alkene with alcohol) is the intermediate. Tautomer mechanism occurs converting OH to ketone and removing the double bond.
*REVIEW MECHANISM OF TAUTOMERIZATION
Markovnikov’s rule is followed in the formation of enol

Question 8

Alkyne + dihalide (Cl2 or Br2)

Answer 8

Halogenation of Alkynes
Adds 2 halides across the alkyne creating an alkene or with 2 equivalences, an alkane.
Addition is anti because of the formation of a halonium ion and attack of the second halide from opposite side.
mechanism same as alkenes

Question 9

Alkyne + O3 + Zn/H2O

Answer 9

Ozonolysis of alkynes

splits triple bond, and adds carboxylic acid to each end. Different from alkenes!

Question 10

Alkene or benzene with allylic carbon + NBS (CCl4 and heat)

Answer 10

Selectively adds Br to an allylic carbon off an alkene or benzene ring

Question 11

Conjugated Diene + hydrohalide (HCl, HBr)

Answer 11

Electrophilic addition to conjugated dienes
H adds to the end of conjugated system. Usually can occur in two pathways, the pathway that produces the two most stable carbocations will predominate, and the other pathway will not occur
Addition can occur in by either 1,2 or 1,4 - 1,4 is the result from addition on the resonance structure.
Produces either kinetic or thermodynamic product.

Question 12

Kinetic Product

Answer 12

Is usually formed by the most stable carbocation, occurs quickest. Usually predominates at low temperatures (-80C) when rearrangement to a less stable carbocation intermediate is not energetically favorable even though a higher energy product results.

Question 13

Thermodynamic Product

Answer 13

Thermodynamic product is the most stable product in which the double bond is most highly substituted. Usually involves rearrangement to less stable carbocation intermediate which requires more initial energy but produces a lower energy product. Thermodynamic products predominate at higher temperatures (25C) where there is enough energy to produce a energetically unfavorable carbocation intermediate.

Question 14

Conjugated diene + alkene or alkyne (dienophile)

Answer 14

Diels-Alder reaction!
Concerted one step mechanism which results in the formation of a cyclohexene
Most reactive dienophiles have an Electron Withdrawing Group (EWG) directly attached to the carbon of the double bond.
EWGs: =O or C=_N
EWGs destabilize the alkene or alkyne within the dienophile making it susceptible to attack.
*Involves syn addition to the Alkene/alkyne therefore, cis/trans relationship of substituents in the dienophile is retained in the cyclohexene product.
*conjugated diene MUST be in CIS FORM
Know Mechanism

Question 15

Toluene

Answer 15

Benzene with CH3 substituent

Question 16

Naphthalene

Answer 16

Two benzene rings that share one double bond

Question 17

Benzaldehyde

Answer 17

Benzene plus aldehyde substituent (C=O H)

Question 18

Benzoic Acid

Answer 18

Benzene ring plus carboxylic acid substituent (C=O OH)

Question 19

Phenol

Answer 19

Benzene ring plus alcohol substituent (-OH)

Question 20

Stryrene

Answer 20

Benzene plus ethylene substituent (-CH=CH2)

Question 21

Aniline

Answer 21

Benzene ring plus Amine substituent (-NH2)

Question 22

Anisole

Answer 22

Benzene ring plus ether substituent (-OCH3)

Question 23

Aromaticity

Answer 23

Based on three factors

1. cyclic structure
2. conjugated
3. Huckle’s Rule (4n+2 = # of pi e-) where n must = a whole number

Question 24

Toluene + Cl2 and light or heat

Answer 24

Free- radical chlorination of toluene
Adds Cl to the benzylic position
*Br2 and light or NBS can be used to add Br at benzylic position.

Question 25

Benzene with alkyl group substituent (-CH3, -CH2R, -CHR2) + Na2Cr2O7 + H2SO4 + H2O + heat

Answer 25

Oxidation at Benzylic carbon
Selectively adds a carboxylic acid to benzylic C – NOT DIRECTLY TO THE RING
Benzylic carbon must have at least one H in order for rxn to proceed.

Question 26

Benzene (with or without substituents) + Na, NH3 and CH3OH

Answer 26

Birch Reduction of Benzene
Only reaction that effects arenes! aromaticity is destroyed resulting in isolated diene.
two double bonds are placed across from eachother within the ring.
If there is a substituent present containing a benzylic carbon, the double bonds will be positioned in a way in which the substituent is attached directly to the double bond.
Reaction stops after one reduction. Is not a Hydrogenation, does not involve H2

Question 27

Benzene + HNO3 + H2SO4

Answer 27

Nitration of Benzene
One H on the Benzene ring is replaced with NO2
Electrophile is NO2 (nitronium ion) O=N(+)=O
1. creation of nitronium ion. HONO2 comes in OH group on N attack H on hydronium ion from H2SO4 to create oxonium ion on the N. Then lone pair on one O attached to the N comes down to create a double bond between the N and O to kick off oxonium ion and create nitronium ion.
2. Attack on nitronium ion by double bond within benzene, breaks aromaticity and creates carbocation on adjacent carbon.
3. H2O attacks H on same carbon as Nitro group, pulls off H and bond reforms double bond to satisfy the adjacent carbocation.

Question 28

Benzene + H2SO4 + heat

Answer 28

Sulfonation of benzene
Replaces H on benzene with SO3H (SO2OH)
electrophile here is sulfur trioxide O=S(+) =O -O(-)
1. Attack by double bond in benzene on sulfur trioxide, adds to one C creates carbocation on other carbon
2. Attack on H by H2O to create double bond and satisfy carbocation
3. protonation of negative sulfur trioxide by H2SO4 (hydronium ion) Negative charge on SO3 attack H on hydronium ion
Results in addition of SO3H to benzene.

Question 29

Benzene + Br2 and FeBr3 (or Cl2 and FeCl3)

Answer 29

Halogenation of Benzene
Adds Br or Cl to benzene in replace on H Requires FeX3 catalyst because benzene does not want to react. Can only leave out catalyst if there is a STRONGLY ACTIVATING substituent present on benzene in which case, can replace catalyst for acetic acid
1. acid-base reaction between Br2 and FeBr3. Br2 attacks Fe creating Br2-FeBr3 complex with positive charge on Br and negative charge on Fe. This complex is more electrophilic than Br2 alone
2. Attack on Br2-FeBr3 complex by double bond within benzene. Attacks terminal Br breaking double bond with Br-FeBr3
FeBr4- is bi-product. Therefore Br is added to one side of double bond, and carbocation is formed at other carbon
3. Attack on H by FeBr4- to create double bond and satisfy carbocation
This adds halide to benzene

Question 30

Benzene + any alkyl Chloride + AlCl3

Answer 30

Friedel-Crafts Alkylation
adds alkyl group in place of H sustituent. Must have alkyl group attached to Cl!
1. AlCl3 acts as an acid to promote the ionization of the alkyl group. Here Cl on alkyl chloride attacks Al in AlCl3 and creates complex in which Cl is positive and Al is negative. Then the bond between the alkyl group and Cl breaks to get rid of the positive charge on Cl and create carbocation on alkyl group. *Within this step, rearrangements within the alkyl group can occur to create a more stable carbocation resulting in the addition of a different alkyl group that expected. (other rxns to prevent this)
2. double bond in benzene attacks the carbocation in the alkyl group. adds to benzene, creating adjacent carbocation.
3. AlCl4- deprotonates the H from the benzene creating a double bond satisfying the carbocation.
To add on long iso- alkyl groups, use acylation then reduce

Question 31

Bezene + any AcylChloride (CH3CH2C=O Cl) + AlCl3

Answer 31

Friedel-Crafts Acylation of benzene
Acyl Chloride (MUST BE CHLORIDE) replaces H for acyl group -- biproduct is HCl
Acyl cation is formed by same method as alkylation in reaction with AlCl3 
1. Attack on acylchloride ion by double bond within the benzene. Adds acylchloride to ring, breaks double bond, creating adjacent carbocation 
2. Deprotonation of H from bezene by AlCl4- to create double bond and satisfy carbocation 
Acylation can be reduced to remove =O and leave alkyl group

Question 32

Acyl group on bezene + Zn(Hg), HCl

Answer 32

Clemmensen reduction of aldehyde and ketone carbonyl groups to result in alkyl group.
Usually useful for the addition of primary alkyl groups to benzylic carbon.
*Careful to not use this acidic reduction on structure that contain other substituents that may react alternatively with Acids! (Ex: alkenes with eliminate, Bases will consume all H’s in solution)

Question 33

Acyl group on benzene + H2NNH2, KOH, heat (triethylene glycol)

Answer 33

Wolff-Kishner reduction of aldehyde and ketone carbonyl groups to produce alkyl group on benzylic position
Useful for the addition of primary alkyl groups to benzylic carbons.
This is used in replacement of clemmensen reduction in cases where acids can not be used. Be careful to avoid using in when subtituents that can react with bases are present. (Ex: -OH will deprotonate and use up all active base not allowing reaction to occur).

Question 34

Activating Substituents are what kind of directors?

Answer 34

Ortho and Para. Usually para predominates due to sterics. The larger the activating substituent (i.e. t-butyl) the more para products will be produced.

Question 35

What type of directors are deactivating substituents?

Answer 35

Meta. The only deactivators that direct ortho/para are halides that are directly attached to the benzene ring. These are weak deactivators. All deactivators inhibit Friedel-Crafts reactions– important to consider in syntheses.

Question 36

Electron Releasing Groups (ERGs)

Answer 36

Activators that are ortho and para directing.
Can include alkyl groups (-R), other arenes (-AR) and -C=C
Also includes substituents that have lones pairs on the atom directly attached to the ring– results in donation of electrons to structure = stabilization
Ex: -OH -OR -OC=O R, -NH2, -NHR, -NR2, -NHC=O R
*These are STRONG ACTIVATORS – therefore the presence of a catalyst in the electrophilic addition reactions is not necessary.
Ex: addition of Br2 to a benzene with -OCH3 does not require FeBr3 to occur, can replace with acetic acid.

Question 37

Electron Withdrawing Groups (EWGs)

Answer 37

Deactivators, direct meta addition.
-CF3 is a STRONG deactivator that withdraws electrons from the benzene ring.
Other strong deactivators include: -C=O H, -C=O R, -C=O OH, -C=O OR, -C=O Cl <— these all have carbonyls attached directly to the ring.
The O pulls electrons away from the structure because it is electronegative and IS NOT DIRECTLY ATTACHED TO THE RING ITSELf – therefore IT DOES NOT DONATE
Other strong deactivators include: -NO2, -SO3H, -C=-N
*Deactivators prevent friedel-Craft rxns.
Halogens are considered weak deactivators and promote ortho/para addition.