Sn1 Reaction – Haloalkanes

by Adam Le Gresley, PhD

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    00:00 although fluoride, less so. Right. Now, we talked about SN2 reactions, but there is the so called SN1 reaction. This is substitution nucleophilic unimolecular.

    00:10 This takes place in the absence that I said of actually an attacking nucleophile and we’ll go through that. But, before I do, I just want to talk about the steric chemistry. If we said, or if we look at the previous SN2 reaction, you can see we get inversion of the steric chemistry for SN2. An S, or sinister, stereochemistry under Cahn-Ingold-Prelog rule is inverted to an R or recto stereochemistry. However, when an SN1 reaction takes place, you can actually convert an enantiomer into a racemic mixture, where you have approximately equal amounts of S and R enantiomers. And that is the basis of an SN1 reaction in terms of the stereochemistry. So, why is that? Let’s have a quick look.

    01:00 In the SN1 mechanism, the reaction coordinate is actually not dependent upon the concentration of your nucleophile. In an SN1 reaction, the loss of halide takes place first, thus generating a carbocation. As you can see, in the center of this particular reaction series, you’ve got here a plain old mirror that show because what you have, as I showed you before in the case of the alkanes, I am sorry, alkenes, is a p-orbital, which is perpendicular to that trigonal planar arrangement. This means, after that carbocation is formed, which is an intermediate, is that the nucleophile, in this case our hydroxide, can attack from either side. By being able to attack from either side, which is a fast step in this particular reaction, you can convert an enantiomerically pure compound into its racemate, which, in fairness, is often not particularly desirable. The reality, when it comes to SN1 reactions, is that you tend to see more of one enantiomer than the other. This is due to the ion pair effect. But, that goes beyond our terms of reference.

    02:15 The reaction is said to be unimolecular, that is, the number 1 in SN1 stands for unimolecular.

    02:22 Only the halogeno-alkane is participating in the slow step of the reaction and therefore, the rate is dependent solely on the concentration of the haloalkane. Therefore, the reaction kinetics are thus where the reaction rate, k, is proportional to the concentration of the haloalkane. Bear in mind, this is just to do with kinetics; nothing to do with thermodynamics.

    02:49 Now, let’s have a look at some of the reaction parameters relating to SN1 reactions, specifically, the nucleophilicity of the nucleophile, the solvent, the alkyl group and the leaving group.

    03:05 Let’s have a look at the stability of the carbocations first because as we indicated, the slow step is actually the formation of the carbocation. The nucleophilicity of the nucleophile is to a large extent irrelevant in this case, since it is the fast step of the reaction that it participates in. Unlike the SN2 reaction, the order of stability or the order of reactivity for this particular mechanism is reversed.

    03:30 As you can appreciate from having looked at the way in which addition reactions take place over alkenes, you can see that a positive charge surrounded by electron donating alkyl groups, shown as R in this particular case, can better stabilise that carbocation and therefore, the conversion of a haloalkane to a carbocation... carbocation by the loss of its halide is more likely to occur for a tertiary haloalkane.

    03:57 The next, and most stable, is the secondary, shown there as 2, primary - 1 and then finally methyl. The methyl group is a very unstable carbocation, due to a slight inductive effect of the alkyl groups that partially stabilise the carbocation.

    04:16 All that was said previously regarding the stability of the nucleophiles is true for SN1 reactions. However, also, to facilitate the charge separation in the first step, a good ionising solvent will be needed. So, this effectively alters our parameter set for the SN1 reaction. So, ideally, what we want is something that stabilises the carbocation in the first case. So, unlike with the SN2, solvents such as water and formic acid and alcohols are generally considered better ionising solvents than some organic solvents such as DMSO and dimethylformamide. So, let’s have a look at these parameters

    About the Lecture

    The lecture Sn1 Reaction – Haloalkanes by Adam Le Gresley, PhD is from the course Organic Chemistry.

    Included Quiz Questions

    1. …an enantiomer into the racemic mixture.
    2. …an S-enantiomer into an R-enantiomer.
    3. …an R enantiomer into an S- enantiomer.
    4. …a primary alkyl halide into a mixture of secondary and tertiary alkyl halides.
    5. …a tertiary alkyl halide into a secondary alkyl halide.
    1. The reaction rate of SN1 type reaction depends upon the concentrations of both alkyl halide and the attacking nucleophile.
    2. The bromide ion of 3-bromo-3-propyloctane leaves at first place during the SN1 reaction; then the incoming nucleophile (OH-) attacks the generated carbocation.
    3. The bromide ion losing-step (unimolecular) is the rate-determining step in above reaction.
    4. The product consists of a mixture of S and R enantiomers of 3-Propyloctan-3-ol.
    5. An intermediate carbocation is generated during SN1 type reaction.
    1. The carbocation intermediate contains negative charge on the carbon atom due to the presence of a lone pair of electrons.
    2. The carbocation intermediate has an sp2 hybridized carbon with the trigonal planar arrangement.
    3. Due to the trigonal planar arrangement of carbocation intermediate, the incoming nucleophile can attack the either side of the carbocation.
    4. The carbocation formation is not favored energetically, hence this is a slow and rate determining step.
    5. The carbocation intermediate has a positively charged carbon atom.
    1. …ion pair effect.
    2. …trigonal planar arrangement of carbon atom of carbocation intermediate.
    3. …high concentration of attacking nucleophile.
    4. …unfavorable temperature conditions of the reaction mixture.
    5. …presence of an impurity or heavy metal in the reaction mixture.
    1. The rate of SN1 reaction depends upon the nucleophilicity of the nucleophile.
    2. The rate of SN1 reaction depends upon the stability of the carbocation intermediate.
    3. With an increase in the alkyl substitution of the positively charged carbon of carbonation, the probability of SN1 reaction increases.
    4. The polar protic solvents usually favor SN1 type reactions in tertiary haloalkanes.
    5. A strong leaving group in haloalkane favors the SN1 type reaction.
    1. …tertiary > secondary > primary > methyl.
    2. …secondary > primary > methyl > tertiary.
    3. …methyl > primary > secondary > tertiary.
    4. …primary > secondary > tertiary > methyl.
    5. …secondary > tertiary > primary > methyl.
    1. Ethanol
    2. Toluene
    3. Benzene
    4. DMSO
    5. THF
    1. The fluoroalkanes give faster SN2 and SN1 type reactions with NaOH in the presence of a non-polar solvent.
    2. The polar protic solvents favor SN1 type reactions because of they solvate the nucleophile by forming a solvent shell around it.
    3. 2-Chloro-2-methylpentane undergoes an SN1 type reaction more readily than 1-Chloro-pentane.
    4. The electron-releasing inductive effect of alkyl groups in a highly substituted haloalkane impacts the stability of carbocation.
    5. 2-Iodo-2-ethyloctane gives a faster SN1 reaction than 2-chloro-2-ethyloctane. as I- is a better leaving group than Cl-.

    Author of lecture Sn1 Reaction – Haloalkanes

     Adam Le Gresley, PhD

    Adam Le Gresley, PhD

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