3.2 Electron-Pair Displacement Reactions

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90 CHAPTER 3 • ACIDS AND BASES. THE CURVED-ARROW NOTATION

The correct application of the curved-arrow notation also involves computing and properly assigning the formal charge to the products. For each reaction involving the curved-arrow notation, the algebraic sum of the charges on the reactants must equal the algebraic sum of the charges on the products. In other words, total charge is conserved. Thus, in Eq. 3.2, the reactants have a net charge of 1; hence, the products must have the same net charge. By calcu- lating the formal charge on- boron and ﬂuorine, we determine that the charge must reside on boron. To illustrate the application of the curved-arrow notation to a Lewis acid–base dissociation

reaction, let’s consider the dissociation of the ion _BF4 to give BF3 and F_; this reaction is the reverse of Eq. 3.2. The curved-arrow notation for this reaction is as follows: F F 332 332 (3.3) FB"_ F F "B F _ 3 2 L 2 3 3 2 L + 3 2 3 2 "F 2 2 "F 2 33 33 Because the B F bond breaks in2 this reaction, this bond is2 the source of the electron pair that is transferred toL a ﬂuorine to give ﬂuoride ion.

PROBLEM 3.1 Use the curved-arrow notation to derive a structure for the product of each of the following Lewis acid–base association reactions; be sure to assign formal charges. Label the Lewis acid and the Lewis base, and identify the atom that donates electrons in each case.

(a) CH3 (b) F 3 2 3 H3C "C| H2O NH3 "BF L + 2 3 2 + L 2 3 "CH3 "F 2 3 3 2

3.2 ELECTRON-PAIR DISPLACEMENT REACTIONS

A. Donation of Electrons to Atoms That Are Not Electron-Deﬁcient In some reactions, an electron pair is donated to an atom that is not electron-deﬁcient. When this happens, another electron pair must simultaneously depart from the receiving atom so that the octet rule is not violated. The following reaction is an example of such a process. destination displaced of displaced destination electron pair donated electron pair of donated electron pair electron pair H H

_ (3.4) HH|"N OH H "N HHO L ++3 2 L 3 2 L "H hydroxide2 ion "H water2 ammonium ion ammonia In this reaction, a hydrogen of the ammonium ion receives an electron pair from the hydroxide ion. As a result, this hydrogen becomes bonded to the oxygen to give water, and the electron pair in the N H bond of the ammonium ion becomes the unshared pair in the product ammonia. If the latterL electron pair had not departed, hydrogen would have ended up with more electrons than allowed by the octet rule. 03_BRCLoudon_pgs5-1.qxd 12/8/08 11:53 AM Page 91

3.2 ELECTRON-PAIR DISPLACEMENT REACTIONS 91

A reaction such as this, in which one electron pair is displaced from an atom (in this case, from a hydrogen) by the donation of another electron pair from another atom, is called an electron-pair displacement reaction. In many such reactions, an atom is transferred between two other atoms. In this example, a proton is transferred from the nitrogen of the ammonium ion to the oxygen of the hydroxide ion.

B. The Curved-Arrow Notation for Electron-Pair Displacement Reactions The curved-arrow notation is particularly useful for following electron-pair displacement reactions. This usage can be illustrated with the reaction of Eq. 3.4. In this case, two arrows are required, one for the donated electron pair and one for the displaced electron pair:

donated electron pair H H

H |"N H OH_ H "N + H O H (3.5) L 3 21 L 3 12 L "H "H

displaced electron pair

Notice that in all uses of the curved-arrow notation, each curved arrow originates at the source of electrons—an unshared pair or a bond—and terminates at the destination of the STUDY GUIDE LINK 3.1 electron pair. The Curved-Arrow Notation Notice also the conservation of total charge on each side of the equation, as discussed in Sec. 3.1C. The algebraic sum of the charges on the left side is zero; hence, the net charge of all species on the right side must also be zero. The donated electron pairs can originate from bonds as well as unshared pairs. This is il-

lustrated by the reaction of _BH4 with water to give dihydrogen (H2) and hydroxide. donated electron pair

H H

" ) HH H _B H H OH H B) _ OH (3.6) L 21 L + + 3 21 "H H

displaced electron pair

In this notation, the bond corresponding to the donated electrons is “hinged” at the transferred atom (the H of the B H bond); it swings away from the boron and towards the atom that receives the electrons (theL H of water). The acceptor atom can be an atom other than hydrogen, as shown in Study Problem 3.2.

Study Problem 3.2 Give the curved-arrow notation for the following reaction. H H

H"C Cl _ OH H"C OH _ Cl LL2 3 + 3 2 LL2 + 3 2 3 "H 2 2 "H 2 2 03_BRCLoudon_pgs5-1.qxd 12/8/08 11:53 AM Page 92

92 CHAPTER 3 • ACIDS AND BASES. THE CURVED-ARROW NOTATION

Solution In this reaction, an unshared electron pair from the oxygen of _OH displaces the electron pair from the C Cl bond onto the chlorine; the carbon atom is transferred from the Cl to the oxygen. Because thisL is an electron-pair displacement reaction, two arrows are required. Remember that a curved arrow is drawn from the source of an electron pair to its destination. The

source of the donated electron pair is the _OH ion. The destination of the donated electron pair is

the carbon atom. Hence, one curved arrow goes from an electron pair of the _OH (any one of the three pairs) to the carbon atom. Because carbon can have only eight electrons, it must lose a pair of electrons to the chloride ion, which is formed in the reaction. Hence, the source of this electron pair is the C Cl bond; its destination is the chlorine. The curved-arrow notation for this reaction is as follows:L H H

H "C Cl H "C OH Cl _ L L 12 3 L L 21 + 3 21 3 "H "H

HO _ 12 3 (Be sure to read Study Guide Link 3.1 about the different ways that curved arrows can be drawn.)

Study Problem 3.2 shows how to write the curved-arrow notation for a completed reaction. Study Problem 3.3 shows how to complete a reaction for which the curved-arrow notation is given.

Study Problem 3.3 Given the following two reactants and the curved-arrow notation for their reaction, draw the structure of the product. H

H3N $C A O ? 2 1 3 CH)3 Solution The bonds or unshared electron pairs at the tails of the arrows are the ones that will not be in the same place in the product. The heads of the arrows point to the places at which new bonds or unshared pairs exist in the product. Use the following steps to draw the product.

Step 1 Redraw all atoms just as they were in the reactants: H

H3N C O

CH3

Step 2 Put in the bonds and electron pairs that do not change: H

H3N $C O L 1 3 CH)3

Step 3 Draw the new bonds or electron pairs indicated by the curved-arrow notation: 03_BRCLoudon_pgs5-1.qxd 12/8/08 11:53 AM Page 93

3.2 ELECTRON-PAIR DISPLACEMENT REACTIONS 93

new electron pair H

H3N $C O L L 21 3 CH)3 new bond

Step 4 Complete the formal charges to give the product. The algebraic sum of the formal charges in the reactants and products must be the same—zero in this case. H

| H3N $C O _ L L 21 3 CH)3

Always remember that curved arrows show the ﬂow of electron pairs, not the movement of nuclei. Beginning students sometimes forget this point. For example, the proton transfer from STUDY GUIDE LINK 3.2 Rules for Use of the HCl to _OH might be incorrectly written as follows: Curved-Arrow Notation INCORRECT CURVED-ARROW NOTATION!

HO _ H Cl HO H _Cl (3.7) 12 3 L 12 3 12 L + 3 12 3 This is incorrect because it shows the movement of the proton rather than the ﬂow of electron pairs. Someone accustomed to using the notation correctly would take this to imply the trans-

fer of H_ to _OH, an impossible reaction! The correct use of the curved-arrow notation shows the ﬂow of electron pairs, as follows:

HO__H Cl HO H Cl CORRECT! (3.8) 21 3 L 21 3 21 L + 3 21 3 As you learn to use the curved-arrow notation, you will ﬁnd the additional assistance in Study Guide Links 3.1 and 3.2 to be very useful. Be sure to read and study these carefully.

PROBLEMS 3.2 For each of the following cases, give the product(s) of the transformation indicated by the curved-arrow notation.

(a) HO_ CH2 Cl (b) (CH3)2CAC(CH3)2 H Br 21 3 L 2 3 L 2 3 "CH3 2 2 O (c) H3Al_ H H3C Br (d) H2C 2S3

LL2 3 A O 2 H2C |3 O) 3 1 _3 3.3 Provide a curved-arrow notation for each of the following reactions in the left-to-right direction. A (a) CH3O_ H CH2 CH Br CH3OHH 2C CH Br _ 2 3 L LL2 3 2 L 3 2 3 2 "CH3 2 2 "CH3 2

| (b) H3NH3C Br H3N CH3 Br _ 33L 2 L 3 2 3 2 2