{"id":5333,"date":"2012-07-04T11:10:10","date_gmt":"2012-07-04T11:10:10","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=5333"},"modified":"2026-04-18T05:55:29","modified_gmt":"2026-04-18T10:55:29","slug":"the-sn2-mechanism","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2012\/07\/04\/the-sn2-mechanism\/","title":{"rendered":"The SN2 Mechanism"},"content":{"rendered":"

The SN<\/sub>2 Reaction Mechanism<\/strong><\/p>\n

Having gone through the two different types of substitution reactions<\/a>, and talked about nucleophiles and electrophiles<\/a>, we’re finally in a position to reveal the mechanism for one of the most important reactions in organic chemistry.<\/p>\n

It’s called the SN<\/sub>2 reaction<\/strong>, and it’s going to be extremely useful for us going forward.<\/p>\n

\"summary<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. The SN<\/sub>2 Reaction Proceeds With Inversion of Configuration<\/a><\/li>\n
  2. The Rate Law Of The SN<\/sub>2 Is Second Order Overall<\/a><\/li>\n
  3. The Reaction Rate Is Fastest For Small Alkyl Halides (Methyl > Primary > Secondary >> Tertiary)<\/a><\/li>\n
  4. The SN<\/sub>2 Mechanism Proceeds Through A Concerted Backside Attack Of The Nucleophile Upon The Alkyl Halide<\/a><\/li>\n
  5. Notes<\/a><\/li>\n
  6. Quiz Yourself!<\/a><\/li>\n
  7. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. The SN<\/sub>2 Reaction Proceeds With Inversion of Configuration<\/strong><\/h2>\n

    When we start with a molecule with a chiral center, such as (S<\/em>)-2-bromobutane, this class of reaction results in inversion of stereochemistry<\/strong>. Note how we start with (S<\/em>)-2-bromobutane and end up with (R<\/em>)-2-methylbutanenitrile.\u00a0<\/span><\/p>\n

    \"sn2<\/p>\n

    <\/a>2. The Rate Law Of The SN<\/sub>2 Is Second Order Overall<\/strong><\/h2>\n

    Note how the rate of the reaction is dependent on both the concentration of the nucleophile and that of the substrate. In other words, it’s a second-order reaction<\/strong>.<\/p>\n

    \"rate<\/p>\n

    <\/a>3. The Reaction Rate Of The SN<\/sub>2 Reaction Is Fastest For Small Alkyl Halides (Methyl > Primary > Secondary >> Tertiary)<\/strong><\/h2>\n

    Finally, note how changes in the substitution pattern of the alkyl halide results in dramatic changes in the rate of the reaction.[Note 1<\/a>] “Smaller” alkyl halides like methyl bromide are fast<\/strong>, while more highly substituted tertiary alkyl bromide doesn’t proceed at all.<\/p>\n

    \"rate<\/p>\n

    Taking all this data into consideration, we refer to this reaction as the SN<\/sub>2 mechanism. What does SN<\/sub>2 stand for?<\/p>\n