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Organic Chemistry

6.3 Polar Reactions

Organic Chemistry6.3 Polar Reactions

6.3 • Polar Reactions

Polar reactions occur because of the electrical attraction between positively polarized and negatively polarized centers on functional groups in molecules. To see how these reactions take place, let’s first recall the discussion of polar covalent bonds in Section 2.1 and then look more deeply into the effects of bond polarity on organic molecules.

Most organic compounds are electrically neutral; they have no net charge, either positive or negative. We saw in Section 2.1, however, that certain bonds within a molecule, particularly the bonds in functional groups, are polar. Bond polarity is a consequence of an unsymmetrical electron distribution in a bond and is due to the difference in electronegativity of the bonded atoms.

Elements such as oxygen, nitrogen, fluorine, and chlorine are more electronegative than carbon, so a carbon atom bonded to one of these atoms has a partial positive charge (δ+). Metals are less electronegative than carbon, so a carbon atom bonded to a metal has a partial negative charge (δ−). Electrostatic potential maps of chloromethane and methyllithium illustrate these charge distributions, showing that the carbon atom in chloromethane is electron-poor (blue) while the carbon in methyllithium is electron-rich (red).

Electrostatic potential maps and wedge-and-dash structures of chloromethane and methyllithium.

The polarity patterns of some common functional groups are shown in Table 6.1. Note that carbon is always positively polarized except when bonded to a metal.

Table 6.1 Polarity Patterns in Some Common Functional Groups
Compound type Functional group structure
Alcohol has a carbon with three open single bonds and a partial positive charge bonded to a hydroxyl group with a partial negative charge on oxygen.
Alkene has two carbons double bonded to each other. Each carbon has two open single bonds. Text reads, symmetrical, nonpolar.
Alkyl halide has a carbon with three open single bonds and a partial positive charge bonded to a halogen with a partial negative charge.
Amine has a carbon with three open single bonds and a partial positive charge bonded to an amine group with a partial negative charge on nitrogen.
Ether has a central oxygen atom with a partial negative charge bonded to two carbon atoms each with a partial positive charge and three open single bonds.
Thiol has a carbon with three open single bonds and a partial positive charge bonded to a thiol group with a partial negative charge on sulfur.
Nitrile has a carbon with an open single bond and a partial positive charge triple bonded to a nitrogen with a partial negative charge.
Grignard reagent has a carbon with three open single bonds and a partial negative charge bonded to M g B r with a partial positive charge on magnesium.
Alkyllithium has a carbon with three open single bonds and a partial negative charge bonded to lithium with a partial positive charge.
Carbonyl has a carbon with two open single bonds and a partial positive charge double bonded to an oxygen with a partial negative charge.
Carboxylic acid has a carbon with an open single bond and a partial positive charge double bonded to oxygen and single bonded to hydroxyl each with a partial negative charge.
Carboxylic acid chloride has a carbon with an open single bond and a partial positive charge double bonded to oxygen and single bonded to chlorine each with partial negative charge.
Thioester has carbon with open single bond and partial positive charge double bonded to oxygen and single bonded to sulfur with partial negative charges. Sulfur is bonded to another carbon.
Aldehyde has a carbon with an open single bond and a partial positive charge single bonded to hydrogen and double bonded to oxygen with a partial negative charge.
Ester has carbon with open bond and partial positive charge double bonded to oxygen and single bonded to another oxygen with partial negative charges. Oxygen is bonded to another carbon.
Ketone has carbon with an open single bond and a partial positive charge bonded to another carbon and double bonded to oxygen with a partial negative charge.

This discussion of bond polarity is oversimplified in that we’ve considered only bonds that are inherently polar due to differences in electronegativity. Polar bonds can also result from the interaction of functional groups with acids or bases. Take an alcohol such as methanol, for example. In neutral methanol, the carbon atom is somewhat electron-poor because the electronegative oxygen attracts the electrons in the C−O bond. On protonation of the methanol oxygen by an acid, however, a full positive charge on oxygen attracts the electrons in the C−O bond much more strongly and makes the carbon much more electron-poor. We’ll see numerous examples throughout this book of reactions that are catalyzed by acids because of the resultant increase in bond polarity upon protonation.

A reaction shows methanol (weakly electron-poor carbon) reacting with H A to form protonated methanol (strongly electron-poor carbon). The electrostatic potential maps of both species are also depicted.

Yet a further consideration is the polarizability (as opposed to polarity) of atoms in a molecule. As the electric field around a given atom changes because of changing interactions with solvent or other polar molecules nearby, the electron distribution around that atom also changes. The measure of this response to an external electrical influence is called the polarizability of the atom. Larger atoms with more loosely held electrons are more polarizable, and smaller atoms with fewer, tightly held electrons are less polarizable. Thus, sulfur is more polarizable than oxygen, and iodine is more polarizable than chlorine. The effect of this higher polarizability of sulfur and iodine is that carbon–sulfur and carbon–iodine bonds, although nonpolar according to electronegativity values (Figure 2.3), nevertheless usually react as if they were polar.

First structure shows carbon (partial positive charge) bonded to sulfur (partial negative charge), which bonds to hydrogen. Second structure shows carbon (partial positive charge) bonded to iodine (partial negative charge).

What does functional-group polarity mean with respect to chemical reactivity? Because unlike charges attract, the fundamental characteristic of all polar organic reactions is that electron-rich sites react with electron-poor sites. Bonds are made when an electron-rich atom donates a pair of electrons to an electron-poor atom, and bonds are broken when one atom leaves with both electrons from the former bond.

As we saw in Section 2.11, the movement of an electron pair during a polar reaction is indicated using a curved, full-headed arrow to show where electrons move when reactant bonds are broken and product bonds are formed during the reaction.

Full arrow points from lone pair on nucleophile species B (negative, electron-rich) to electrophile species A (positive, electron-poor) to form species A B with single bond.

In referring to the electron-rich and electron-poor species involved in polar reactions, chemists use the words nucleophile and electrophile. A nucleophile is a substance that is “nucleus-loving.” (Remember that a nucleus is positively charged.) A nucleophile has a negatively polarized, electron-rich atom and can form a bond by donating a pair of electrons to a positively polarized, electron-poor atom. Nucleophiles can be either neutral or negatively charged; ammonia, water, hydroxide ion, and chloride ion are examples. An electrophile, by contrast, is “electron-loving.” An electrophile has a positively polarized, electron-poor atom and can form a bond by accepting a pair of electrons from a nucleophile. Electrophiles can be either neutral or positively charged. Acids (H+ donors), alkyl halides, and carbonyl compounds are examples (Figure 6.2).

Electrostatic potential maps of nucleophiles ammonia, water, hydroxide ion, and chloride ion, and electrophiles hydronium ion, methyl bromide, and a carbonyl group.
Figure 6.2 Some nucleophiles and electrophiles. Electrostatic potential maps identify the nucleophilic (negative) and electrophilic (positive) atoms.

Note that neutral compounds can often react either as nucleophiles or as electrophiles, depending on the circumstances. After all, if a compound is neutral yet has an electron-rich nucleophilic site, it must also have a corresponding electron-poor electrophilic site. Water, for instance, acts as an electrophile when it donates H+ but acts as a nucleophile when it donates a nonbonding pair of electrons. Similarly, a carbonyl compound acts as an electrophile when it reacts at its positively polarized carbon atom, yet acts as a nucleophile when it reacts at its negatively polarized oxygen atom.

If the definitions of nucleophiles and electrophiles sound similar to those given in Section 2.11 for Lewis acids and Lewis bases, that’s because there is indeed a correlation. Lewis bases are electron donors and behave as nucleophiles, whereas Lewis acids are electron acceptors and behave as electrophiles. Thus, much of organic chemistry is explainable in terms of acid–base reactions. The main difference is that the words acid and base are used broadly in all fields of chemistry, while the words nucleophile and electrophile are used primarily in organic chemistry when carbon bonding is involved.

Worked Example 6.1

Identifying Electrophiles and Nucleophiles

Which of the following species is likely to behave as a nucleophile and which as an electrophile?

(a) NO2+(b) CN(c) CH3NH2(d) (CH3)3S+

Strategy

A nucleophile has an electron-rich site, either because it is negatively charged or because it has a functional group containing an atom that has a lone pair of electrons. An electrophile has an electron-poor site, either because it is positively charged or because it has a functional group containing an atom that is positively polarized.

Solution

(a) NO2+ (nitronium ion) is likely to be an electrophile because it is positively charged.

(b):C≡N:C≡N (cyanide ion) is likely to be a nucleophile because it is negatively charged.

(c) CH3NH2 (methylamine) might be either a nucleophile or an electrophile, depending on the circumstances. The lone pair of electrons on the nitrogen atom makes methylamine a potential nucleophile, while positively polarized N−H hydrogens make methylamine a potential acid (electrophile).

(d) (CH3)3S+ (trimethylsulfonium ion) is likely to be an electrophile because it is positively charged.

Problem 6-2
Which of the following species are likely to be nucleophiles and which electrophiles? Which might be both?
(a)
CH3Cl
(b)
CH3S
(c)
The figure shows a five-membered ring with nitrogens at positions 1 and 3, double bonds at N1 and C4, and a methyl group at N3.
(d)
The chemical structure of ethanal.
Problem 6-3

An electrostatic potential map of boron trifluoride is shown. Is BF3 likely to be a nucleophile or an electrophile? Draw a Lewis structure for BF3, and explain your answer.

The electrostatic potential map of B F 3 shows a ball-and-stick model at the center, in which gray and green spheres represent boron and fluorine atoms, respectively.
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