CH0003 - Lecture 4 - Halo-organics

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  1. What is alkane halogenation?
    The reactions between alkanes and cycloalkanes with the halogens fluorine, chlorine, bromine and iodine.

    Alkyl halides can be prepared from the reaction of an alkane with a halogen (e.g. Cl2)

    But aren’t alkanes really unreactive?

    A mixture of Cl2and methane (for example) will not react unless it is initiated.

    This can be achieved with UV light or heating.

    CH4(g) + Cl2(g) ---> CH3(g) + HCl(g)
  2. What are free radicals and how do they feature in alkane halogenation?
    An example of a free radical reaction

    Definition: A (free) radical is an atom, molecule, or ion that has unpaired valence electrons

    Represented as a ‘dot’ e.g. Cl·

    • How is the radical formed?
    • In this context, a covalent bond is split (by UV light) with each atom keeping one electron of the pair that is shared in the covalent bond. HOMOLYTIC FISSION
  3. What is the initiation of alkane halogenation?
    Reaction initiation:

    Cl-Cl(g) --> 2Cl·(g)

    Cl-Cl bond is broken equally by a photon of UV light, forming 2 neutral chlorine free radicals

    Free radicals are highly reactive species which is why they can react with the unreactive C-H bonds of Alkanes

    Once some free radicals have been produced, the reaction is self sustaining i.e. a chain reaction.
  4. What is reaction propagation?
    Reaction propagation:

    Cl·(g) + H-CH3(g) ---> ·CH3(g) + HCl(g) 

    ·CH3(g) + Cl-Cl(g) ---> Cl-CH3(g) + Cl·(g)
  5. What eventually happens with alkane halogenation free radical reaction propagation?
    The free radical reaction propagation produces many chlorinated species, it will continue to produce CCl4if there is enough chlorine available.

    The product distribution can be controlled by carefully controlling the ratio of the reactants.

    What happens if two free radicals combine with each other? This does happen and is called termination
  6. What is termination of alkane halogenation?
    • Reaction termination:
    • Cl·(g) + Cl·(g) ---> Cl2(g)
    • ·CH3(g) + Cl·(g) ---> Cl-CH3(g)

    ·CH3(g) + ·CH3(g) ---> H3C-CH3(g) (ethane!)

    Termination can lead to the reaction stopping if all of the free radicals are consumed.

    Termination yields some very different products such as ethane. These can also undergo chlorination.

    The total number of products is endless, theoretically……

    As mentioned previously product distribution can be controlled by reactant stoichiometry
  7. What is alcohol substitution? Give example
    As shown previously alcohols can react with phosphorus halides to produce the corresponding alkyl halide.

    This can also be done with strong acids i.e. the halo acids, HCl, HBr and HI. (HF is not very good at this - weak acid)

    3 R-OH + PCl3 ---> 3 R-Cl + P(OH)3
  8. What is the mechanism of primary alcohol substitution?
    1st step is activation of the alcohol as –OH is a poor leaving group.

    2. Protonation (if use HX) or reaction with PX3

    3. The activated species is then attacked by the nucleophilic halide
  9. What is the mechanism for the substitution of a tertiary alcohol?
    1. 1st step is activation of the alcohol as –OH is a poor leaving group.

    2. Protonation (if use HX) or reaction with PX3

    3. The activated species dissociates to form a carbocation

    4. Only then does the halide (nucleophile) attack the carbocation (electrophile)

    • The rds is the formation of the carbocation; SN1 (the rate limiting step) is unimolecular.
    • SN2 reactions can’t occur because of the three alkyl groups, very bulky – ‘steric hinderance’
  10. What is the mechanism for the substitution of a secondary alcohol?
    Secondary alcohols can undergo SN1 and SN2 reactions, depending on reaction conditions.

    i.e. a polar solvent will help in the formation of charged intermediates

    e.g. carbocations (by solvating them and making them more stable)
  11. What does a primary alcohol SN2 substitution reaction look like in formula?
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  12. What does a tertiary alcohol SN1 substitution reaction look like in steps?
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  13. What does a primary alcohol SN2 substitution reaction look like in steps?
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  14. What does a tertiary alcohol SN1 substitution reaction look like in steps?
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  15. How do primary and tertiary alcohols follow different reaction mechanisms?
    Primary carbocations are not stable

    Tertiary centres are hindered from attack – steric hindrance
  16. What is an alkyl carbocation?
    • An alkyl carbocation is a carbocation that has the structural formula of an alkyl group.
    • e.g.
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  17. What is a carbocation?
    A carbocation is a species containing a carbon atom that lacks an octet of valence electrons and bears a formal charge of +1.

    • e.g
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  18. Comment on cation stability?
    The stability of alkyl carbocations is shown by theseries:

    R3C+ > R2HC+ > RH2C+

    3o> 2o> 1o

    So… 1alcohols do not form a carbocation, the nucleophile (halide in this case) attacks theactivated intermediate directly, bimolecular SN2

    This explains why 2alcohols can do both –‘intermediate stability’ and intermediate sterichindrance
  19. How else can alkyl halides be synthesised?
    By the addition of HX (halide acids) and X2 (halogens) to alkenes.

    The addition of HBr yields a monobromo product

    The addition of Br2yields a dibromo-product

    The mechanisms and reasons for the substitution pattern using HBr will be explained later on in the course.
  20. What use are halo-organics?
    Alkyl halides are very useful in organic synthesis, they are reactive and easily allow a molecule to be further functionalised (SN1/2)

    Grignard formation

    Alkyl lithium reagent formation

    Carbocation and carbanion (C-) source

    Source of free radicals! Ozone depletion
  21. What are primary alkyl halides?
    In a primary (1°) halogenoalkane, the carbon which carries the halogen atom is only attached to one other alkyl group.
  22. What are secondary alkyl halides
    In a secondary (2°) halogenoalkane, the carbon with the halogen attached is joined directly to two other alkyl groups, which may be the same or different.
  23. What are tertiary alkyl halides?
    In a tertiary (3°) halogenoalkane, the carbon atom holding the halogen is attached directly to three alkyl groups, which may be any combination of same or different.
  24. Why are carbon-halogen bonds polar?
    With the exception of iodine, all of the halogens are more electronegative than carbon.

    That means that the electron pair in the carbon-halogen bond will be dragged towards the halogen end, leaving the halogen slightly negative (-) and the carbon slightly positive (+) - except in the carbon-iodine case.
  25. What happens in the nucleophilic substitution of bromoethane (ethyl bromide) (primary alkyl halide)
    We'll look at its reaction with a general purpose nucleophilic ion which we'll call Nu-. This will have at least one lone pair of electrons. Nu- could, for example, be OH- or CN-.

    The lone pair on the Nu- ion will be strongly attracted to the + carbon, and will move towards it, beginning to make a co-ordinate (dative covalent) bond.

    In the process the electrons in the C-Br bond will be pushed even closer towards the bromine, making it increasingly negative.

    The movement goes on until the -Nu is firmly attached to the carbon, and the bromine has been expelled as a Br- ion.

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  26. Why is there a different mechanism for nucleophilic substitution of tertiary alkyl halides?
    You will remember that when a nucleophile attacks a primary halogenoalkane, it approaches the + carbon atom from the side away from the halogen atom.

    With a tertiary halogenoalkane, this is impossible. The back of the molecule is completely cluttered with CH3 groups.

    Since any other approach is prevented by the bromine atom, the reaction has to go by an alternative mechanism.
  27. What is the mechanism for the nucleophilic substitution of a tertiary alkyl halide?
    The reaction happens in two stages. In the first, a small proportion of the halogenoalkane ionises to give a carbocation and a bromide ion.

    • This reaction is possible because tertiary carbocations are relatively stable compared with secondary or primary ones. Even so, the reaction is slow.
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    • Once the carbocation is formed, however, it would react immediately it came into contact with a nucleophile like Nu-. The lone pair on the nucleophile is strongly attracted towards the positive carbon, and moves towards it to create a new bond.
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    • How fast the reaction happens is going to be governed by how fast the halogenoalkane ionises. Because this initial slow step only involves one species, the mechanism is described as SN1 - substitution, nucleophilic, one species taking part in the initial slow step.
  28. Why don't primary halogenoalkanes use the SN1 mechanism?
    • If a primary halogenoalkane did use this mechanism, the first step would be, for example:
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    • A primary carbocation would be formed, and this is much more energetically unstable than the tertiary one formed from tertiary halogenoalkanes - and therefore much more difficult to produce.

    This instability means that there will be a very high activation energy for the reaction involving a primary halogenoalkane. The activation energy is much less if it undergoes an SN2 reaction - and so that's what it does instead.
  29. Nucleophilic substitution in secondary halogenoalkanes
    There isn't anything new in this. Secondary halogenoalkanes will use both mechanisms - some molecules will react using the SN2 mechanism and others the SN1.

    The SN2 mechanism is possible because the back of the molecule isn't completely cluttered by alkyl groups and so the approaching nucleophile can still get at the + carbon atom.

    The SN1 mechanism is possible because the secondary carbocation formed in the slow step is more stable than a primary one. It isn't as stable as a tertiary one though, and so the SN1 route isn't as effective as it is with tertiary halogenoalkanes.
  30. What is an elimination reaction?
    An imperfect world – Perfect reactions are rare

    With SN1/2 reactions of alkyl halides there are competing reactions

    These reactions are called elimination reactions

    Again, these can be sub-classified into E1and E2

    Elimination reactions of an alkyl halide is when the substrate eliminates a molecule of HX (the acid) – the reverse reaction of addition to an alkene

    • For example:
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    The 2-bromopropane has reacted to give an alkene - propene.
  31. What is the general form of the E1 mechanism? (tertiary alkyl halides)
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    • B: = base
    • X = leaving group (halide)

    In the E1 mechanism, the the first step is the loss of the leaving group, which leaves in a very slow step, resulting in the formation of a carbocation.

    The base then attacks a neighboring hydrogen, forcing the electrons from the hydrogen-carbon bond to make the double bond.

    Since this mechanism involves the formation of a carbocation, rearangements can occur.
  32. What is an example of an E1 reaction? (tertiary alkyl halides?)
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  33. What is the general form of the E2 mechanism? (Primary alkyl halides)
    General form of the E2  mechanism

    In the E2 mechanism, a base abstracts a proton neighboring the leaving group, forcing the electrons down to make a double bond, and, in so doing, forcing off the leaving group.

    When numerous things happen simultaneously in a mechanism, such as the E2 reaction, it is called a concerted step.
  34. What is an example of an E2 reaction?
    Example of the E2 mechanism
  35. Which elimination reactions do secondary alkyl halides do?
  36. Summarise Substitution elimination
    With SN1/2 reactions of alkyl halides there are competing reactions E1/2

    Nucleophiles that are strong bases (NaOH, NaOEt) will favour elimination for 2o and 3o alkyl halides. Substitution will predominate for 1oalkyl halides

    Nucleophiles that are weak bases (-CN, NH3, -Cl,-Br, -I) will result in nucleophilic substitution and the competing side reaction will be E1
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CH0003 - Lecture 4 - Halo-organics
2014-03-21 14:35:10
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