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Ochem > Ch9 > Flashcards

Ch9 Flashcards

0
Q

How do you make an alkoxide?

A

Deprotonate an alcohol with a strong base.

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1
Q

What is the conjugate base of an alcohol?

A

Alkoxide

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2
Q

What is the salt needed to make an ether?

A

Alkoxide.

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3
Q

What is an especially good base for forming alkoxide?

A

NaH because it forms H2 and bubbles out of the reaction mixture.

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4
Q

What sort of reaction is used to make an alkoxide?

A

A Bronsted Lowry acid-base reaction.

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5
Q

What is a halohydrin?

A

Organic compound that contains both a hydroxy group and a halogen on adjacent carbons.

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6
Q

An intramolecular version of the Williamson ether synthesis forms epoxides in this type of organic compound.

A

Halohydrins

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7
Q

How many steps is epoxide synthesis?

A

Two steps.

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8
Q

For an alcohol to undergo nucleophilic substitution what must happen?

A

The hydroxy group must be converted into a better leaving group.

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9
Q

How do you turn the bad leaving group of an alcohol into a good leaving group?

A

Protonate it. Making H2O, an excellent leaving group.

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10
Q

In terms of acid strength, which side does equilibrium lie on?

A

The side with the weaker acid.

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11
Q

How do we protonate the OH group to make it a excellent leaving group?

A

By adding a strong acid. A strong acid with a pKa greater than -2.

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12
Q

What type of reaction is dehydration?

A

Beta elimination.

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13
Q

What occurs during beta elimination?

A

Step one: protonation of oxygen. Resulting in a good leaving group.
Step two: the leaving group leaves resulting in water and a Carbocation.
Step three: Deprotonation of methyl group results in a double bond.

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14
Q

The more substituted the alcohol the more easily it?

A

Dehydrates. Dehydration increases from primary to secondary to tertiary.

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15
Q

What happens with regard to dehydration when an alcohol has two or three beta carbons?

A

Dehydration is regioselective and follows the Zaitsev rule.

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16
Q

When is the more substituted alkene the major product?

A

When a mixture of constitutional isomers is possible.

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17
Q

What is the significance of the E1 dehydration reaction occurring without any byproducts?

A

There is no good nucleophile to react with the intermediate carbocation, so no competing Sn1 reaction occurs.

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18
Q

In dehydration entropy favors the products or the reactants?

A

The products, because one molecule of reactant forms two molecules of product.

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19
Q

In dehydration, enthalpy favors the products or reactants?

A

Enthalpy favors the reactants, since the two sigma bonds broken in the reactant are stronger than the sigma and pi bonds formed in the products.

20
Q

What is a carbocation rearrangement?

A

When a less stable carbocation is converted into a more stable carbocation by a shift of a hydrogen or an alkyl group.

21
Q

How do we know that a rearrangement occurred?

A

There may be a product formed that has the double bond in an unexpected location.

22
Q

Are rearrangements unique to dehydration reactions?

A

No. Rearrangements can occur whenever a carbocation is formed as a reactive intermediate. Like 1,2-shifts.

23
Q

A secondary carbocation likes to rearrange to which carbocation?

A

Tertiary, because it is more stable.

24
Q

Dehydration by phosphorus oxychloride and pyridine proceeds by which mechanism?

A

E2

25
Q

Substitution reactions do not occur with alcohols unless what?

A

The hydroxide is converted into a good leaving group.

26
Q

What is the general method to prepare primary secondary and tertiary alkyl halides?

A

The reaction of alcohols with HX.

27
Q

The most rapidly reacting with HX is?

A

The more substituted alcohol.

28
Q

The reactivity of hydrogen halides increases with?

A

Acidity.

29
Q

Nucleophilicity increases as we go which way on the column?

A

Down.

30
Q

Primary alcohols only react with HCL if this is added.

A

Zinc chloride.

31
Q

Complexation of zinc chloride with oxygen of the alcohol does what?

A

Makes a very good leaving group for Sn2 reaction.

32
Q

Primary alcohol Sn2+ HX leads to?

A

Inversion.

33
Q

Tertiary alcohol in Sn1+ HX leads to?

A

Racemic.

34
Q

SOCL2 and Br3 do what two things to Primary and secondary alcohols?

A

Convert OH into a good leaving group in situ as well as provide the nucleophile to displace the leaving group via Sn2.

35
Q

What are the three elements of the Oxonium ion?

A

Three bonds, one lone pair, positive charge.

36
Q

What sort of leaving group is tosylate?

A

Tosylate is a good leaving group.

37
Q

Alcohols are converted to tosylates by treatment with what in the presence of what?

A

They are treated with TsCl in the presence of pyridine.

38
Q

What happens when ethers react with HBr or HI?

A

Both carbon-oxygen bonds are cleaved and two alkyl halides are formed as products.

39
Q

Do epoxides contain a good leaving group?

A

No.

40
Q

How are epoxide reactions made possible?

A

Nucleophilic attack opens the strained three membered ring, making the reaction favorable even with a poor leaving group.

41
Q

Name five common nucleophiles that open the epoxide ring.

A
  • OH, -OR,
  • SR, -CN
  • NH3
42
Q

Which carbon does the nucleophile attack in an asymmetrical epoxide?

A

The less substituted carbon.

43
Q

What are the products when an achiral reactant yields a product with stereogenic centers?

A

The products must be achiral (meso) or racemic.

44
Q

What is mechanism 9.1?

A

Dehydration of secondary and tertiary alcohols. E1 mechanism.

45
Q

What are the steps of mechanism 9.1: dehydration of secondary and tertiary alcohols. E1 mechanism?

A

1: protonation of the OH group to make it a good leaving group.
2 & 3: E1 mechanism.

46
Q

What are the steps of mechanism 9.2: dehydration of a primary alcohol. E2 mechanism?

A

1: protonation.
2: E2 mechanism.

47
Q

In an unsymmetrical epoxide the nucleophile attacks at which card?

A

The less substituted carbon.

48
Q

With asymmetrical epoxides nucleophilic attack occurs at which carbon?

A

?

Ochem (3 decks)

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