Chem 261 > Reactions > Flashcards

Reactions Flashcards

(52 cards)

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1
Q

What reactions forms three membered rings?

A

Simmons Smith
alkanes → cyclopropanes

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2
Q

What is a carbenoid?

A

C w/ two unshared e- but no FC

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3
Q

What kind of reaction is the Simmons Smith?

A

Stereospecific addition of a CH2 group DIRECTLY to the double bond of an alkene. Stereospecific in that it PRESERVES initial stereochemistry.

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4
Q

Three things.

What conditions does the Simmons Smith reaction take place under?

A

(ZnCu) CH2I2 in Et reflux

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5
Q

Briefly describe the mechanism of the Simmons Smith reaction.

A

CH2I2 + ZnCu strirred with an alkene → carbenoid (on the alkene)

Carbenoid will react in stereospecific way with CH2 group.

The stereochemistry of the double bond is preserved.
i.e in cyclohexene, the Hs are on the same face. So the CH2 group will add in such a was that keeps the two Hs cis.

Conversely when there is a straight chain alkene substituted on either side of the double bond in a trans way, the CH2 will add to add in such a way that preservs the trans stereochemistry of the initial alkene.

The top of the triangle is the CH2 group.

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6
Q

When do you use Z and E notation?

A

When you are naming complex compounds in which the substituents are not the same.

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7
Q

Explain how to assign seniority when using E and Z

A

split compound into LS/RS.

The senior groups are the ones that have the most C’s.

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8
Q

Define E and Z

A

E - Senior groups on opposite sides

Z- Senior groups on zame zide

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9
Q

When do you not use E and Z notation?

A

In naming small rings, and when priority cannot be assigned to either the left side or right side. E.g ls has same substituents.

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10
Q

Why do addition reactions occur?

A

There is a nucleophile and an electrophile

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11
Q

What kind of reaction is hydrogenation of an alkene?

A

SYNaddition of hydrogens directly to the double bond of an alkene.

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12
Q

What are the conditions of a hydrogenation reaction?

A

H2 + metal catalyst
(Pd, Pt, nor Ni)

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13
Q

Briefly describe the mechanism of a hydrogenation

A

An alkene is treated with H2 and a metal catalyst. Hydrogens add to the same face of the double bond. (SYN addition of hydrogens to double bond)

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14
Q

Why do electrophilic reactions occur?

A

Electrophiles (s+) are attracted to the electrons in the π bond of alkene.

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15
Q

Define nucleophiles and electrophiles

A

Nucleophiles - electron donors
e.g anions are nucleophiles
Electrophiles - electron seeking
e.g cations are electrophiles

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16
Q

What kind of reaction is the reaction of a halogen (BR2, CL2, I2, F2) and an alkene?

A

Halogenation. An electrophilic addition

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17
Q

What are the conditions of a reaction of an alkene with a halogen (Br2)

A

Alkene is treated with Br2 in dark

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18
Q

Describe the mechanism of a halogenation of an alkene. (use Br2)

A

Momentary dipole in (Br2) causes the negatively charged π region to become attracted to the + charged Br.

One of the Br will attach to the double bond and form EITHER a CARBOCATION OR a BROMONIUM ION leaving a negatively charged Br-.

In the case of the bromonium ion: Br- will attack one carbon, and open the ring, leaving the first Br on the other side.

In the case of the carbocation: The Br- will attack the carbocation on the other side of the first Br.or will attack one of t

THE ADDITION OF HALOGEN X2 TO ALKENE IS ALWAYS ANTI

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19
Q

What is Markovnikov’s rule?

A

IN adding H-X to an alkene, the H adds to the LEAST substituted carbon (the one with the most Hs). (usually terminal)

  • this will form carbocation*

The halide (X) will then add to the other carbon of the double bond (more substitued)

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20
Q

Markovnikov’s rule results in the formation of what kind of carbocation?

A

Secondary carbocation. More stable.

If Markovnikov’s rules is not followed, a primary carbocation may be formed. ( primary carbocation is terminal)

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21
Q

Describe the mechanism of the addition of H-X to an alkene.

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A

The negatively charged π region of the dbl bond will attract the H, which is electrophilic.

The H will bond to the C of dbl bond with most H (markovnikov)
leaving the other c of double bond as a carbocation.

The X- anion will then bind to the carbocation.

22
Q

How can we generalize markovnikov’s rule?

Study These Flashcards
A

The S+ atom will add to the C of double bond with most hydrogens.

23
Q

what are the two types of acid catalyzed hydration?

Study These Flashcards
A

w/ cold concentratred H2SO4
w/ dilute H2SO4

24
Q

Describe the mechanism of electrophilic reaction with an alkene and COLD CONCENTRATED SULFURIC ACID (H2SO4)

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A

The H on the terminal OH H2SO4 is S+, and will bond to C of double bond following Markovnikov’s rule.

Carbocation and HSO4- are produced.

The HSO4 - will bind to carbocation.

25
Describe the mechanism of electrophilic reaction with an alkene and DILUTE SULFURIC ACID (H2SO4)
The H on the terminal OH H2SO4 is S+, and will bond to C of double bond following Markovnikov's rule. Carbocation and HSO4- are produced. Because the H2SO4 is dilute, water is also present. Water will react with the carbocation instead of the HSO4-. The O of the H2O is S- and will bind to carbocation. (here the oxygen will have a POSITIVE FC, but still draw e density from terminal H towards it, making it have a s- charge. So if youre drawing the lewis structure you need to indicate the + on the O) The result is a protonated alcohol. Since the partial - on O density and partial+ on H. Another water molecule (- on the o) will steal an H resulting in an alcohol and a H3O+.
26
What happens when an electrophilic addition of H-Cl/Br happens in water?
A chlorohydrin, or bromohydrin is produced. (also called a halo alcohol)
27
What is a bromo/chlorohydrin?
compound that has OH and Br/Cl
28
Briefly describe the rxn mechanism for an alkene reacting with H-Br in water.
(-) region on π is attracted to momentary (+) dipole in Br2. A bromonium ion will form (with + charge on bromine) leaving behind a Br- A water will attack a C in the bromonium ion and cause the ring to open. Now there is + FC on oxygen, but still s- on O and s+ on H. Another water will steal one of the Hs making H3O+ and leaving behind an OH group. * THIS IS AN ANTI ADDITION, Br ON ONE END, OH ON OTHER. * there is additional mechanism, review it in the notes, its simple.
29
Describe the structure of an organic peroxide.
R-O-O-R (org peroxide) R-O-O-H (org hyperperoxide)
30
What happens when alkenes containing organic peroxides/hydroperoxides react with HBr?
Anti-markovnikov addition occurs. The H will add to the more substituted C of the double bond, making a less stable primary carbocation. The Br will bond to this carbocation.
31
The mechanism for anti-markovnikov addition of H-Br in the presence of peroxides is a radical chain reaction. Describe the steps. (chain initiation, chain propagation, chain termination etc...)
Chain initiation: 1. Heat cleaves the weak O-O bond and produces two alkoxy radicals 2. The alkoxy radical steals H from H-Br resulting in a Br radical Chain propagation: 3. Br radical adds to alkene double bond making secondary alkyl radical (The Br adds to terminal so anti-m) 4. Alkyl radical steals an H from H-Br, which regenerates a Br radical and leads to the anti-Markovnikov product. (since the Br bonded terminally) Chain termination 5. 2 Br radicals meet or two alkoxy radicals meet (see how to draw this)
32
What is hydroboration? What does it produce?
SYN addition reaction where an alkene reacts with diborane (BH3). Produces a trialkylborane. (syn- new H and B add to the same face of double bond) Oxidation of the trialkylborane. follows to yield an alcohol.
33
Under what conditions can hydroboration take place?
diborane (BH3) dissolved in tetrahydrofuran (THF) You can show this w/ arrow as:
34
Briefly describe the mechanism of hydroboration to yield a trialkylborane.
Start with BH3 and alkene. Boron less EN than H. So S+ on B. On the alkene, S- on The boron will attach to C of dbl bond that is the least substituted (anti-M) producing BH2 - alkane The B attaches terminally since its bulky so more room. The BH2 - alkane will then find another alkene, and the B will bond to the terminal end of this alkene, producing alkene - BH- alkene. This will happen again for the third alkene. This is ANTI MARKOVNIKOV because the B bonds to the C with the most Hs, and the new H will bond to the more substituted c of double bond (less H) note that the new hydrogen and the boron add to the same face of the double bond.
35
What are the three reactions that can follow the formation of a trialkyl borane? (from notes)
1. Treat (CH3-CH2-CH2)3B with aq acetic acid (CH3COOH) → CH2CH2CH3 ( propane) 2. Treat (CH3-CH2-CH2)3B with aq deuteroacetic acid (CH3COOH) → CH2CH2CH2-D 3. Treat (CH3-CH2-CH2)3B with H2O2 and OH- → CH2CH2CH2-OH (propan-1-ol)
36
Hydroboration followed by oxidation yields what? How does this happen?
Following hydroboration, oxidation will replace the BH2 group where it stands with an OH group, giving an alcohol.
37
In hydroboration-oxidation reaction what does the OH directly replace in oxidation?
OH group replaces the BH2 group where it stands.
38
What is the syn 1,2-dihydroxilation reaction of an alkene?
Oxidation reaction where the C=C is oxidized. results in a SYN addition to dbl bond.
39
What are the conditions for syn 1,2-dihydroxilation to take place?
Done with osmium tetroxide (**OsO4**) or potassium permangante (**KMnO4**)
40
Briefly describe the mechanism of syn 1,2-dihydroxilation of an alkene with OsO4.
OsO4 approaches the alkene and two of the Oxygens will form bonds to the Cs of the double bond. Cleavage of the O-Os bond follows. (H2S is needed) The new oxygens on the alkene (now alkane) will bond to Hs. **The result is a cis 1,2-diol.** (each c of the double bond now has an OH on it -- these OH groups are on the same side, which is why its syn and produces a cis-1,2-diol)
41
Briefly describe the mechanism of syn 1,2-dihydroxilation of an alkene with KMnO4.
KMnO4 approaches the alkene and two of the Oxygens will form bonds to the Cs of the double bond. Cleavage of the O-Mn bond follows. (**OH- is needed**) The new oxygens on the alkene (now alkane) will bond to Hs. **The result is a cis 1,2-diol.** (each c of the double bond now has an OH on it -- these OH groups are on the same side, which is why its syn and produces a cis-1,2-diol)
42
What is the structure of ozone (O3)
O=O-O + -
43
What happens when you treat an alkene with ozone?
The double bond is cleaved, and you get aldehydes and or ketones.
44
Briefly describe the mechanism for cleavage of a double bond with ozone.
The two terminal Os of ozone add to the Cs of the double bond and makes a ring. The C=C is cleaved. resulting in one half of the moleucle being = to O, and the other half of themoelcule being = to O-O. (One half gets one O, theo ther half gets 2 O). These fragments recombine to form another ozonide with the two Cs stacked above two Os, and an O on top. This ozonide is then treated with various products which splits it into an aldehyde or a ketone (usually)
45
What is a peracid?
an organic peroxy acid. RCO3H
46
What happens to the double bond when an alkene is treated with a peracid?
The double bond is oxidized.
47
What is the product of oxidation of an alkene with a peracid?
An epoxide. A cyclic ether with three membered ring. (the o is the top of the triangle). you also get the acid, but with one less oxygen.
48
What is the stereochemistry of a reaction with an peracid?
The reaction is stereospecific. the stereochemistry of the alkene is preserved. The O can add to either face of the double bond but wont change the stereochemistry.
49
What is an example of a peracid and its abbreviation?
meta-chloroperbenzoic acid mCPBA
50
What is an ionic addition
When markovnikov addition occurs. (H attacks double bond and bonds to C of double bond with the most Hs in it). Happens with HCl, HI, HF, and HBr in the absence of peroxides.
51
What is a radical addition?
When anti-M addition occurs. Happens only with Br in the presence of peroxides.
52
Chem 261
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