{"id":4309,"date":"2018-07-30T12:45:41","date_gmt":"2018-07-30T12:45:41","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/?post_type=chapter&p=4309"},"modified":"2018-08-09T21:15:09","modified_gmt":"2018-08-09T21:15:09","slug":"7-6-common-elementary-steps","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/7-6-common-elementary-steps\/","title":{"raw":"7.6. Common elementary steps","rendered":"7.6. Common elementary steps"},"content":{"raw":"Although there there are many different mechanisms, there are just a few common elementary steps that make up those mechanisms.\u00a0 It is useful to examine those in more detail, and to try and recognize each one when it occurs, as this makes it easier to see the similarities between mechanisms.\r\n

Acid-base<\/h2>\r\nThis is based on the Bronsted-Lowry definition of acid and base, where an H+ is transferred from one atom to another.\u00a0 We have already seen examples of this mechanism in section 6.3.<\/a>\u00a0 It is perhaps the commonest of all the elementary steps.\u00a0 When talking about elementary steps, we limit the use of this term to those cases where only sigma bonds are broken and made, as here where water acts as a base to remove an H+<\/sup> from a protonated alcohol:\r\n\r\n\"\"\r\n\r\nOften H3<\/sub>O+<\/sup> (i.e., aqueous acid) serves as the acid, as here:\r\n\r\n\"\"\r\n

Bimolecular nucleophilic substitution: SN<\/sub>2<\/h2>\r\nWe will study the SN<\/sub>2 reaction in depth in the next chapter.\u00a0 Since it is a single-step (concerted) reaction, the reaction is itself an elementary step.\u00a0 The electron flow is a pair of arrows - identical to that of the acid-base, except that the nucleophile is attacking a carbon (or sometimes other elements), not hydrogen.\r\n\r\n\"\"\r\n

Coordination<\/h2>\r\nCoordination involves the direct bond formation that occurs when a nucleophile attacks an electrophile with an incomplete octet, such as a carbocation:\r\n\r\n\"\"\r\n

Heterolysis<\/h2>\r\nHeterolysis involves breaking a sigma bond in such a way that both electrons leave with one atom.\u00a0 It is the opposite of coordination, and it results in the formation of a molecule with an incomplete octet, usually a carbocation:\r\n\r\n\"\"\r\n

Bimolecular elimination: E2<\/h2>\r\nAs with SN2, this is an example of an elementary step that is also the name of the complete one-step reaction; we will study this reaction in depth in section 8.5.\u00a0 Being an elimination, a new pi bond must be formed.\u00a0 The E2 elementary step involves three curved arrows: The base attacks, forms a new pi bond, and the leaving group leaves, all at the same time:\r\n\r\n\"\"\r\n

Nucleophilic addition<\/h2>\r\nIn nucleophilic addition, a nucleophile attacks a pi bond and breaks it, forming a new sigma bond from the attacking atom.\u00a0 Being an addition, a pi bond must be broken.\u00a0 It is a common elementary step in carbonyl chemistry, where often a C=O (or its protonated form) is attacked by a nucleophile:\r\n

\"\"<\/h2>\r\n

Nucleophile elimination<\/h2>\r\nThis is the reverse of nucleophilic addition, and thus it results in a nucleophilic group being expelled.\u00a0 In the example given below, the water is the nucleophile being expelled.\u00a0 Being an elimination, a new pi bond must be formed, often a C=O or C=N:\r\n\r\n\"\"\r\n

Electrophilic addition<\/h2>\r\nHere, a pi bond attacks an electrophilic atom to form a new sigma bond.\u00a0 Being an addition, a pi bond must be lost:\r\n\r\n\"\"\r\n\r\nThe electrophile may in fact be H+<\/sup>, in which case the addition may be confused with an acid-base step; note that if a pi bond is lost it should be classified as an electrophilic addition, as here:\r\n\r\n\"\"\r\n

Electrophile elimination<\/h2>\r\nThis is the reverse of electrophilic addition; in this elementary step an electrophile is ejected and a new pi bond is formed.\u00a0 As with electrophilic addition, the step may be confused with acid-base if the ejected electrophile is based on H+, but if a new pi bond is formed it must be classed as an elimination:\r\n\r\n\"\"\r\n

Carbocation rearrangements<\/h2>\r\nThese are a more specialized elementary step, but they are mentioned here because we will study these rearrangements in section 8.4.<\/a>\u00a0 There are two common types, hydride shifts and alkyl shifts, where an H or alkyl respectively migrates towards a C+, and leaves a carbocation on the carbon it left.\u00a0 The example shows a typical hydride shift:\r\n\r\n\"\"\r\n

These videos cover the common elementary steps in detail:<\/h2>\r\n\"\"\r\n\r\n[embed]https:\/\/www.youtube.com\/watch?v=MQSFOzLPQyQ[\/embed]\r\n\r\n\"\"\r\n\r\n[embed]https:\/\/www.youtube.com\/watch?v=WwPHT17_uQI[\/embed]\r\n

References<\/h3>\r\nJoel Karty, \"Organic Chemistry: Principles and Mechanism,\" Chapter 7, First Edition, Norton.","rendered":"

Although there there are many different mechanisms, there are just a few common elementary steps that make up those mechanisms.\u00a0 It is useful to examine those in more detail, and to try and recognize each one when it occurs, as this makes it easier to see the similarities between mechanisms.<\/p>\n

Acid-base<\/h2>\n

This is based on the Bronsted-Lowry definition of acid and base, where an H+ is transferred from one atom to another.\u00a0 We have already seen examples of this mechanism in section 6.3.<\/a>\u00a0 It is perhaps the commonest of all the elementary steps.\u00a0 When talking about elementary steps, we limit the use of this term to those cases where only sigma bonds are broken and made, as here where water acts as a base to remove an H+<\/sup> from a protonated alcohol:<\/p>\n

\"\"<\/p>\n

Often H3<\/sub>O+<\/sup> (i.e., aqueous acid) serves as the acid, as here:<\/p>\n

\"\"<\/p>\n

Bimolecular nucleophilic substitution: SN<\/sub>2<\/h2>\n

We will study the SN<\/sub>2 reaction in depth in the next chapter.\u00a0 Since it is a single-step (concerted) reaction, the reaction is itself an elementary step.\u00a0 The electron flow is a pair of arrows – identical to that of the acid-base, except that the nucleophile is attacking a carbon (or sometimes other elements), not hydrogen.<\/p>\n

\"\"<\/p>\n

Coordination<\/h2>\n

Coordination involves the direct bond formation that occurs when a nucleophile attacks an electrophile with an incomplete octet, such as a carbocation:<\/p>\n

\"\"<\/p>\n

Heterolysis<\/h2>\n

Heterolysis involves breaking a sigma bond in such a way that both electrons leave with one atom.\u00a0 It is the opposite of coordination, and it results in the formation of a molecule with an incomplete octet, usually a carbocation:<\/p>\n

\"\"<\/p>\n

Bimolecular elimination: E2<\/h2>\n

As with SN2, this is an example of an elementary step that is also the name of the complete one-step reaction; we will study this reaction in depth in section 8.5.\u00a0 Being an elimination, a new pi bond must be formed.\u00a0 The E2 elementary step involves three curved arrows: The base attacks, forms a new pi bond, and the leaving group leaves, all at the same time:<\/p>\n

\"\"<\/p>\n

Nucleophilic addition<\/h2>\n

In nucleophilic addition, a nucleophile attacks a pi bond and breaks it, forming a new sigma bond from the attacking atom.\u00a0 Being an addition, a pi bond must be broken.\u00a0 It is a common elementary step in carbonyl chemistry, where often a C=O (or its protonated form) is attacked by a nucleophile:<\/p>\n

\"\"<\/h2>\n

Nucleophile elimination<\/h2>\n

This is the reverse of nucleophilic addition, and thus it results in a nucleophilic group being expelled.\u00a0 In the example given below, the water is the nucleophile being expelled.\u00a0 Being an elimination, a new pi bond must be formed, often a C=O or C=N:<\/p>\n

\"\"<\/p>\n

Electrophilic addition<\/h2>\n

Here, a pi bond attacks an electrophilic atom to form a new sigma bond.\u00a0 Being an addition, a pi bond must be lost:<\/p>\n

\"\"<\/p>\n

The electrophile may in fact be H+<\/sup>, in which case the addition may be confused with an acid-base step; note that if a pi bond is lost it should be classified as an electrophilic addition, as here:<\/p>\n

\"\"<\/p>\n

Electrophile elimination<\/h2>\n

This is the reverse of electrophilic addition; in this elementary step an electrophile is ejected and a new pi bond is formed.\u00a0 As with electrophilic addition, the step may be confused with acid-base if the ejected electrophile is based on H+, but if a new pi bond is formed it must be classed as an elimination:<\/p>\n

\"\"<\/p>\n

Carbocation rearrangements<\/h2>\n

These are a more specialized elementary step, but they are mentioned here because we will study these rearrangements in section 8.4.<\/a>\u00a0 There are two common types, hydride shifts and alkyl shifts, where an H or alkyl respectively migrates towards a C+, and leaves a carbocation on the carbon it left.\u00a0 The example shows a typical hydride shift:<\/p>\n

\"\"<\/p>\n

These videos cover the common elementary steps in detail:<\/h2>\n

\"\"<\/p>\n