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CH0003 > Halogenoalkanes > Flashcards

Halogenoalkanes Flashcards

1
Q

What is a halogenoalkane

A

An organic compound that contains a halogen

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2
Q

What is the general formula of mono-halogenoalkanes

A

CnH2n+1X

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3
Q

How do you name halogenoalkanes

A

Same as alkanes but with halogeno-prefix

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4
Q

What is the name of CH3CH2F

A

Fluoroethane

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5
Q

What is the name of CH3CHBrCHBrCHICCl3

A

3,4-Dibromo-1,1,1-trichloro-2-iodopentane.

alphabetic order

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6
Q

How are halogenoalkanes classified as primary, secondary or tertiary

A

According to the number of carbon atoms binding to the carbon atom that the halogen is attached to (like alcohols)

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7
Q

What are the physical properties of halogenoalkanes

A

They are subject only to van der Waals forces, but they have permanent dipoles so the attraction is stronger than in alkanes.

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8
Q

Why do halogenoalkanes have higher boiling points that alkanes

A

Because halogenoalkanes have permanent dipoles so the attraction is stronger

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9
Q

Which halogens giver higher boiling points

A

Larger halogens

I>Br>Cl>F

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10
Q

Why is F the worst group to leave

A

It is around the same size as H, C-F bonds are strong so it is the worst group to leave

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11
Q

Why is the size of the halogen a determinate of the reactivity

A

The larger the halogen the weaker the X-C bonds so the better a leaving group so the more reactive

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12
Q

How can halogenoalkanes be formed

A
  1. ) From alcohols.
  2. ) By electrophilic addition.
  3. ) By free radical substitution.
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13
Q

What is free radical substitution

A

A chain reaction that involves initiation, propagation and termination

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14
Q

What reaction happens during initiation

A

Cl2 -> 2Cl·

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15
Q

What is homolytic fission and when does it occur

A

Homolytic fission is where one electron goes to each of the molecules previously bound. Happens in initiation (one electron goes to each Cl)

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16
Q

What reactions happen during propagation

A

Cl· + H-CH3 -> ·CH3 + HCl

Then ·CH3 + Cl2 -> CH3Cl + Cl·

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17
Q

What reactions occur during termination

A

2Cl· -> Cl2
and ·CH3 + Cl· -> CH3Cl
and 2·CH3 -> CH3CH3

18
Q

During which stages of initiation, propogation and termination can the product of the reaction be formed

A

Propagation and termination

19
Q

What is the basis of ozone depletion by CFCs

A

The C-Cl bond is easily broken by UV light:

CH3-Cl (+uv) -> ·CH3 + Cl·

20
Q

What is the name of CHCl3 and give a use of it

A

Chloroform (trichloromethane). Makes a good anaesthetic

21
Q

What does electrophilic addition of HX to alkenes involve

A

Involves the protonation of an alkene then the attack of a halogen

22
Q

Draw the mechanism of electrophilic addition of HX to alkenes

A

Goes to the more substituted form

23
Q

How do alkyl groups help to stabilise carbocations

A

They spread the positive charge

24
Q

What is the hierarchy of stability of carbocations

A

Primary > Secondary > Tertiary

25
Q

Which carbocation is the most stable

A

Tertiary

26
Q

What is a carbocation

A

A carbon with a positive charge

27
Q

Draw a primary carbocation

A

ethyl

28
Q

Draw a secondary carbocation

A

2-propyl

29
Q

Draw a tertiary carbocation

A

Diagram

30
Q

How do you form a halogenoalkane from alcohols

A

Treat the alcohol with PX3 or with HX (HCl, HBr, HI)

31
Q

What is Sn2

A

Bimolecular nucleophilic substitution

32
Q

Describe the mechanisms of the reaction of a primary alcohol by Sn2

A

The alcohol is protonated (fast). protonated alcohol is attacked by Cl- (nucleophile) forms halogenoalkane and water (slower). The Sn2 part of the reaction is when the protonated alcohol is attacked by a nucleophile

33
Q

What are the molecules involved in the rate determining steps of Sn2

A

Protonated alcohol and halogen nucleophile

34
Q

What are the two simultaneous processes of Sn2

A

Attack with alcohol and loss of water

35
Q

What does the rate of the Sn2 reaction depend on

A

The concentrations of both the alcohol and the acid

36
Q

How do primary alcohols react

A

By Sn2

37
Q

How do tertiary alcohols react

A

By Sn1

38
Q

Why do tertiary alcohols react by Sn1

A

Because the central carbon is not accessible to nucleophilic attack

39
Q

Describe the mechanism of a tertiary alcohol reacting by Sn1

A

A proton attacks the OH group on the tertiary alcohol (fast). The C-O bond breaks, a water molecule and tertiary carbocation form (very slow). The carbocation is attacked by a nucleophile (halogen) due to the shape of the carbocation making it accessible (fast). A halogenoalkane is formed.

40
Q

What does the rate of reaction depend on

A

The concentration of the alcohol as only alcohol is present in the slow step.

41
Q

What does the nucleophile attack in Sn2

A

Accessible δ+C

42
Q

What does the nucleophile attack in Sn1

A

A stabilised carbocation (C+)

CH0003 (12 decks)

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