{"id":3755,"date":"2018-07-16T20:35:48","date_gmt":"2018-07-16T20:35:48","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/?post_type=chapter&p=3755"},"modified":"2020-06-23T15:01:16","modified_gmt":"2020-06-23T15:01:16","slug":"7-3-types-of-reaction","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/7-3-types-of-reaction\/","title":{"raw":"7.3. Types of reaction","rendered":"7.3. Types of reaction"},"content":{"raw":"We will now look at the reaction level in more detail.\u00a0 There are several ways to classify organic reactions, but it is helpful here for is to organize reactions in terms of the overall process that occurs.\u00a0 There are three main types: Substitution, elimination and addition.\r\n

Substitution<\/h1>\r\nIn this process, we simply replace one group (X) with another (Y).\u00a0 We will initially be studying nucleophilic<\/strong> <\/em>substitution (SN<\/sub>2 and SN<\/sub>1, chapters 8 & 9), where the \"replacing\" group Y is a nucleophile.\u00a0 However, we can also have electrophilic substitution, where Y is an electrophile, and we will study this next semester (in aromatic substitution reactions).\r\n\r\n\"\"\r\n\r\nNotice that here we are working at the reaction level, looking at the overall change in the reaction<\/em><\/strong>, not the details of what happens at the mechanism level.\u00a0 For example, next semester we study the acyl substitution reaction, where the mechanism involves an addition followed by an elimination, leading to a substitution overall.\r\n\r\nA typical example of nucleophilic substitution would be this SN<\/sub>2 reaction, where the Br (leaving group) is replaced by the OH (an\u00a0incoming nucleophilic group):\r\n\r\n\"\"\r\n\r\nwhich may be written in scheme format as\r\n\r\n\"\"\r\n\r\nWe can summarize substitution in terms of the overall bond changes: We have broken one sigma bond (R-X or C-Br) and replaced it with a new sigma bond (R-Y or C-O).\r\n

Elimination<\/h1>\r\nHere, an organic molecule eliminates groups X and Y (which may leave either separately or together) to form a new pi bond.\r\n\r\n\"\"\r\n\r\nThe reaction may be driven by another molecule, such as an acid or base.\u00a0 In the next chapter we will learn about two mechanisms that lead to formation of an alkene by an overall elimination reaction: E2 and E1, which are assisted by another molecule.\u00a0 In other cases a different pi bond may be formed, such as C=O.\u00a0 In this example E2 elimination, the formation of an alkene is driven by a base (hydroxide ion):\r\n\r\n\"\"\r\n\r\nwhich may be written as a scheme\r\n\r\n\"\"\r\n\r\nOverall, we break two sigma bonds and form a new pi bond.\u00a0 [A new sigma bond is also formed but is hidden, because it is the H-OH bond formed to produce the by-product, water (not shown).]\r\n

Addition<\/h1>\r\nAddition is the opposite of elimination: X and Y add across a pi bond and form two new sigma bonds.\r\n\r\n\"\"\r\n\r\nIn chapter 10 we will learn about a common type of addition called electrophilic addition, where an alkene reacts with an electrophile.\u00a0 Next semester we will study nucleophilic additions to the pi bond of a carbonyl group (C=O).\u00a0 In the following electrophilic addition example, X and Y are in the same molecule, HBr, which is the electrophilic agent.\u00a0 This reaction can be written either as a balanced equation or a scheme:\r\n\r\n\"\"\r\n\r\n\"\"\r\n\r\nNote that overall we lose a pi bond, and we form two new sigma bonds.\u00a0 [Also, a sigma bond was broken but not shown explicitly, in H-Br bond of the reagent.]\r\n
\r\n

Common types of reaction<\/h3>\r\n