Electrophiles, Nucleophiles and Leaving Groups

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Among the four halide ions, fluoride, chloride, bromide and iodide, fluoride is the strongest base. So it should also be the best at attacking a carbon, right? Well, in water or in an alcohol, it is the worst of the four! And in a solvent like DMSO, it becomes the best. That sounds strange, doesn't it? Today we meet electrophiles, nucleophiles and leaving groups. This comes up often in JEE, so let's begin.

0:34Electrophiles: the electron-seekers

So, first, the electrophile. The name means electron-seeking: it goes looking for electrons. So tell me, does an electrophile accept an electron pair, or donate one? It accepts a pair, yes, exactly! The symbol on screen, E plus, is a capital E with a plus charge written high. And anything that accepts an electron pair is a Lewis acid, so every electrophile is a Lewis acid. Okay? Now the charged examples: H plus, the proton, then N O two plus, B r plus, R plus, where R is an alkyl group, and RCO plus. The small two written low is a subscript. So tell me, must every electrophile carry a positive charge? No, it need not, and here is why. Look at B F three: boron has only six valence electrons, an incomplete octet, so it can accept a pair. Aluminium chloride, written A l C l three, then S O three, and carbenes are neutral electrophiles too. And the carbon of C double bond O is polarised, so it is electrophilic; the angle on its left shows its two other bonds. With me?

1:44Nucleophiles: the nucleus-seekers

Now its partner, the nucleophile. Its name means nucleus-seeking, and it is written N u minus, with a minus charge written high. It donates an electron pair, so it is a Lewis base. Right? The anions first: O H minus, hydroxide, C N minus, cyanide, R O minus, an alkoxide, and X minus, a halide ion, where the letter X stands for a halogen. And carbanions too, okay? Then neutral molecules with lone pairs or pi bonds; the Greek letter pi marks a pi bond. Water, H two O, ammonia, N H three, alcohols, ROH, amines, RNH two, and alkenes, which attack with their pi electrons. Got it? Next, an ambident nucleophile can attack through two different atoms. Cyanide, C N minus, attacks through carbon or nitrogen; nitrite, N O two minus, through nitrogen or oxygen; and thiocyanate, SCN minus, through sulphur or nitrogen. Clear?

2:46Nucleophilicity versus basicity

Now, basicity and nucleophilicity sound alike, but they measure different things. Basicity measures how far an equilibrium with H plus, the proton, goes. Okay? Nucleophilicity measures how fast a species attacks a carbon. So basicity is about how far, and nucleophilicity is about how fast. Right? Now tell me, O H minus or water, which one is the better nucleophile? It is O H minus, yes, exactly! An anion is a better nucleophile than its conjugate acid; here the greater than sign means better nucleophile than. So O H minus beats water, N H two minus beats ammonia, and R O minus beats ROH, okay? Next, when the attacking atom is the same, oxygen every time here, nucleophilicity follows basicity. So R O minus, then H O minus, then phenoxide, C six H five O minus, then acetate, C H three COO minus, and water last. With me? And across a period, it follows basicity again. C H three minus, then N H two minus, then O H minus, then F minus: carbon, nitrogen, oxygen and fluorine, left to right. Clear?

3:59Down a group, the solvent decides

Now the tricky part. Go down a group, from oxygen to sulphur, or from fluorine to iodine, and the bases get weaker. So nucleophilicity must fall too, right? Oh no, careful, not in a protic solvent like water or an alcohol! There, nucleophilicity increases with size, as we go down the group. Okay? The reason is solvation: the small ions are tightly solvated, held by the solvent molecules. That is why R S minus, with sulphur, beats R O minus, with oxygen, in a protic solvent. Right? But in a polar aprotic solvent, like DMSO, DMF or acetone, the halide order is reversed: fluoride becomes the best. Hold on to this, because example two puts it to work. Got it? Now here is one more trap. This is tert butoxide: the brackets with the small three mean three methyl groups on one carbon, joined to the O minus. It is a strong base, but a poor nucleophile, because it is so bulky. Clear?

5:04Leaving groups

Now the third player, the leaving group, the group that leaves the carbon. The rule is simple: a good leaving group is a weak base, the conjugate base of a strong acid. Okay? So, among the halide ions, which one is the best leaving group? It is iodide, that is correct! Iodide is the weakest base of the four, so it leaves best. So the order is iodide, bromide, chloride, and fluoride last, each written with its symbol and a minus charge. Right? Tosylate, written T s O minus, and water are good leaving groups. But O H minus, R O minus and N H two minus are poor ones, because they are strong bases. So the strong nucleophile O H minus is a poor leaving group. With me?

5:50Example 1: classify NO2+, BF3, NH3 and five more species

Alright, example one. Classify each as an electrophile or a nucleophile: N O two plus, C N minus, B F three, N H three, H three O plus, C two H five O minus, aluminium chloride, and water. Pause the video and try it first. Done? Okay, let's check. The ions are easy, but B F three, aluminium chloride, ammonia and water carry no charge. So they are neither electrophiles nor nucleophiles, right? No, no, wait, a charge is not needed! N O two plus and H three O plus are positive, and H three O plus is a source of H plus. B F three and aluminium chloride have incomplete octets, so they accept a pair. So all four are electrophiles. Okay? And the nucleophiles: C N minus and C two H five O minus, ethoxide, are anions, while ammonia and water have lone pairs to donate. So those four are the nucleophiles. Right?

6:53Example 2: halide nucleophilicity in methanol vs DMSO

Now, remember our puzzle from the start? Let's settle it. Arrange the halide ions in order of decreasing nucleophilicity, first in methanol, and then in DMSO, parts A and B on screen. Pause the video and try it first. Done? Okay, let's check part A, methanol. Fluoride is the strongest base of the four, so it must be the best nucleophile, right? Oh no, careful, that is the trap! Methanol is a protic solvent, and the small fluoride ion is held by hydrogen bonds to the solvent. It is tightly solvated, so it attacks poorly. Okay? So in methanol, the order is iodide, then bromide, then chloride, and fluoride last; here, bigger is better. Got it? Now part B, where DMSO has no hydrogen bonding to hold the ions. So the order simply follows basicity: fluoride, then chloride, then bromide, and iodide last. The order is completely reversed! So our puzzle is solved: in methanol, fluoride is the worst nucleophile, and in DMSO, it is the best. Clear?

8:03Revision: Lewis acid-base, nucleophilicity, leaving groups

So, quick revision. An electrophile accepts an electron pair and is a Lewis acid; a nucleophile donates one and is a Lewis base. For the same attacking atom, or across a period, nucleophilicity follows basicity; down a group, the solvent decides. And a good leaving group is a weak base, like iodide, tosylate or water; O H minus is poor. Now try the practice problems on this page. All the best!