7.6 Electrophilic Addition and Electrophile Elimination Steps

An electrophilic addition step occurs when a species containing a nonpolar π bond (as part of a double or triple bond) approaches a strongly electron-deficient species—an electrophile—and a bond forms between an atom of the π bond and the electrophile (Equations 7-18 and 7-19).

Two chemical reactions are shown to represent an example of electrophilic addition step. The first reaction shows two carbon atom linked by a triple bond, with each of the carbon atom linked with a methyl group to each of the group by a single bond, labeled non-polar pi-bond, reacting with hydrochloride acting as an electrophile. A curved arrow is drawn from a triple bond between two carbon atoms with its head pointing toward a hydrogen atom of hydrochloride. Another curved arrow is drawn from a single bond between the hydrogen and chlorine atom with its head pointing toward chloride. The resultants show an addition of hydrogen to one of the carbon atoms with the other carbon atom carrying a positive charge. It also shows a release of chlorine anion carrying four lone pairs of electrons. The second reaction shows a closed six-ring structure having an alternate double bond, labeled as non-polar pi-bond, reacting with NO 2, acting as an electrophile. The nitrogen atom of the NO 2 is shown carrying a positive charge. The resultant shows an addition of nitrogen dioxide to the second carbon atom of the closed six-ring and a positive charge on the first carbon atom of the closed ring.

The nonpolar π bonds involved in electrophilic addition steps are typically ones that join a pair of carbon atoms. The electrophile, on the other hand, can have a variety of different forms. For example, the electrophile can be H+ from a Brønsted acid, such as HCl in Equation 7-18. Alternatively, the electrophile can exist on its own, as shown for the species in Equation 7-19.

The product of each of these electrophilic addition steps is a carbocation, which is highly unstable and will react further because it has a positive charge and lacks an octet. Electrophilic additions, therefore, are generally part of multistep mechanisms. Equation 7-18, for example, is the first step in the electrophilic addition of an acid across a multiple bond (Chapters 11 and 12). Equation 7-19, on the other hand, is the first step in electrophilic aromatic substitution (Chapters 22 and 23).

YOUR TURN 7.10
SHOW ANSWERS

Add the appropriate curved arrows for the following electrophilic addition step.

A chemical reaction represents an electrophilic addition step to draw appropriately curved arrows. It shows a six-carbon zigzag chain, with methyl groups on the extreme ends, and a double bond between carbon 3 and 4 reacting with hydrogen bromide, with the bromine atom in hydrogen bromide carrying three lone pairs of electrons. The resultant shows the addition of a hydrogen atom to the third carbon atom, replacement of the double bond with the single bond, and an addition of a positive charge on the fourth carbon atom of the chain. It also shows a release of bromine anion carrying four lone pairs of electrons.

The curved arrow originates from the electron-rich double bond and points to the electron-poor H of H—Br.

A chemical reaction represents an electrophilic addition step to draw appropriately curved arrows. It shows a six-carbon zigzag chain, with methyl groups on the extreme ends, and a double bond between carbon 3 and 4 which is a electron rich group reacting with hydrogen bromide, with the bromine atom in hydrogen which is interacting with the double bonds in the carbon atoms 3 and 4 and bromide carrying three lone pairs of electrons. The single bonded atom which is an electron poor group is interacting with the bromine atom. The resultant shows the addition of a hydrogen atom to the third carbon atom, replacement of the double bond with the single bond, and an addition of a positive charge on the fourth carbon atom of the chain. It also shows a release of bromine anion carrying four lone pairs of electrons.

Connections Nitrobenzene (C6H5NO2, Equation 7-21) is primarily used in the production of aniline, which is a precursor to a variety of compounds, such as explosives, dyes, and pharmaceutical drugs. Nitrobenzene has an odor that resembles almonds, making it useful in the fragrance industry.

A perfume bottle, with a pump to spray.

Carbocations are typically quite unstable, so the reverse of electrophilic addition is also a common elementary step in organic reactions. In the reverse step, called electrophile elimination, an electrophile is eliminated from the carbocation, generating a stable, uncharged, organic species. Equations 7-20 and 7-21 show examples in which H+ is the electrophile that is eliminated.

Two chemical reactions are shown to represent the second step of E1 reaction and exemplify the process of electrophile elimination. The first reaction shows a condensed structural formula with a central carbon atom surrounded by two methyl groups and a CH 2 group by a single bond each. The CH 2 group is further linked to a hydrogen atom by a single bond. A curved arrow points from a single bond between CH and hydrogen atom toward a single bond between a carbon atom and CH 2. The resultant shows a release of a hydrogen ion carrying a positive charge and another compound with a central carbon atom linked to two methyl groups by a single bond each, and a CH 2 group by a double bond. The second reaction shows a condensed structural formula of a compound as a closed six-ring structure, with the first carbon atom and a presence of two double bonds between carbon-2 and 3, and carbon-4 and 5 each. A hydrogen atom and a nitrogen dioxide are linked to the fifth carbon atom of the ring by a single bond each. A curved arrow from the single bond between carbon and hydrogen on the fifth carbon atom is shown to point toward a single bond between carbon-5 and 6. The resultant shows a release of a hydrogen ion carrying a positive charge and another compound showing a closed six-ring structure, with alternate double bonds, and presence of nitrogen dioxide on the fifth carbon atom linked by a single bond.

Equation 7-20 is the second step of an E1 reaction (Chapter 8) and Equation 7-21 is the second step of an electrophilic aromatic substitution reaction (Chapters 22 and 23).

YOUR TURN 7.11
SHOW ANSWERS

Add the appropriate curved arrow(s) to the following electrophile elimination step, which is essentially the reverse of the addition step in Your Turn 7.10.

A chemical reaction represents an example of an electrophile elimination step. The chemical reaction shows a six-carbon zigzag chain, with methyl groups on the extreme ends, a hydrogen atom linked to the third carbon atom by a single bond and a positive charge at the fourth carbon atom. The resultant shows an elimination of a hydrogen atom linked to the third carbon atom and the replacement of a single bond between carbon-3 and 4 by a double bond. It also shows a release of a hydrogen ion marked with a positive charge.

To show the C—H bond breaking, a curved arrow originates from the center of the C—H bond. To show the pair of electrons ending up in the C═C double bond, the curved arrow points to the center of the C—C bond.

A chemical reaction represents an example of an electrophile elimination step. The chemical reaction shows a six-carbon zigzag chain, with methyl groups on the extreme ends, a hydrogen atom linked to the third carbon atom by a single bond and a positive charge at the fourth carbon atom which is a electron poor group. The single bond in the third carbon and hydrogen bond is interacting with the fourth carbon bond which is an electron rich group. The resultant shows an elimination of a hydrogen atom linked to the third carbon atom and the replacement of a single bond between carbon-3 and 4 by a double bond. It also shows a release of a hydrogen ion marked with a positive charge.

Even though Equations 7-20 and 7-21 show that H+ is the electrophile that is eliminated, a proton does not exist on its own in solution. Rather, it must be associated with a base. Any base that is present in solution will therefore assist in the removal of a proton in an electrophile elimination step. If water is present, for example, then Equation 7-20 would more appropriately be written as follows in Equation 7-22:

A chemical reaction represents an example of an electrophile elimination step. The reaction shows a water molecule with its oxygen atom carrying two lone pairs of electrons reacting with a compound having a central carbon atom surrounded by two methyl groups linked by a single bond each, and a CH 2 group by a single bond. The CH 2 group is further linked to a hydrogen atom by a single bond. A curved arrow points from the oxygen atom of the water molecule toward hydrogen atom linked to CH 2 group while another curved arrow points from the single bond between CH 2 and hydrogen atom toward single bond between a central carbon atom and CH 2 group. The resultant shows an addition of a hydrogen atom to the oxygen atom of the water molecule carrying a lone pair of electrons with a positive charge. It also shows a release of a compound as a central carbon atom surrounded by two methyl groups linked by a single bond each, and a CH 2 group by a double bond.

As shown in Equation 7-23, the electrophile (E+) in an electrophilic addition step is relatively electron poor because it either carries a full positive charge (Equation 7-19) or has an atom with a significant partial positive charge (Equation 7-18). The double or triple bond, on the other hand, is relatively electron rich. In a C═C double bond, for example, four electrons are localized in the region between two atoms, and in a C≡C triple bond, six electrons are localized between two atoms. Therefore, the movement of electrons from electron rich to electron poor is indicated by a curved arrow that originates from the center of the multiple bond and terminates at the electrophile.

A chemical reaction shows an example of an electrophilic addition step emphasizing on the origin and termination of a curved arrow to represent the transfer of electrons. It shows two carbon atoms linked by a double bond, with each carrying two vacant single bonds and labeled electron-rich. It is shown to react with E carrying positive charge and labeled electron-poor. The resultant shows an addition of E to the first carbon atom and replacement of double bond between two carbon atoms by a single bond. The second carbon atom is shown carrying a positive charge.

In electrophile elimination (Equation 7-24), the positively charged C atom is relatively electron poor, whereas the C—E single bond is relatively electron rich. Therefore, the curved arrow originates from the C—E single bond and points to the bond between C and C+.

A chemical reaction shows an example of an electrophilic elimination step to check for potent electron-rich and electron-poor sites. It shows a chain of two carbon atoms linked to each other by a single bond, with each carbon atom carrying two vacant single bonds. The first carbon atom is marked with a positive charge and labeled electron-poor while the second carbon atom is linked to an E group by a single bond and labeled electron-rich. The resultant shows two carbon atoms linked by a double bond, with each carrying two vacant single bonds and a release of E carrying a positive charge.

YOUR TURN 7.12
SHOW ANSWERS

In both Your Turn 7.10 and 7.11, label the pertinent electron-rich and electron-poor sites.

The labels are provided in the answers to Your Turns 7.10 and 7.11.

problem 7.16 Supply the appropriate curved arrows and draw the product of each of the following electrophilic addition steps.

Two chemical reactions to find the product of electrophilic addition steps. The first chemical reaction shows a closed six-ring structure carrying alternate double bonds reacting with a bent-shaped, three-carbon chain, with the second carbon atom carrying a positive charge. It is followed by a rightward arrow to read a question mark. The second reaction shows a chain of six-carbon atom with two bents on the extreme ends, and a long chain carrying a double bond in the middle, reacting with HBr. It is followed by a rightward arrow to read a question mark.

problem 7.17 Supply the appropriate curved arrows and draw the product of each of these electrophile elimination steps. (You may assume that a weak base is present.)

Two chemical reactions to identify the potent curved arrows in electrophile elimination steps. The first chemical reaction shows a closed six-ring structure with two double bonds, one between carbon-1 and 2 while another between carbon-5 and 6. There are two empty side chains marked at carbon-1 and 6. A positive charge is marked at carbon-3, followed by a rightward arrow to read a question mark. The second reaction shows a closed six-ring structure, with alternate double bonds. A side chain of two carbon atoms is linked to the sixth carbon of the chain, with its first carbon carrying a positive charge, followed by a rightward arrow to read a question mark.