Reactions of the O–H Bond

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Drop a piece of sodium into ethanol, and bubbles of hydrogen gas come off. So ethanol behaves like an acid, right? But dip litmus paper into ethanol, and it does not turn red. And sodium hydroxide solution cannot turn ethanol into sodium ethoxide. So, is ethanol an acid, or not? Today we study the reactions in which the O H bond of an alcohol breaks. Questions on them come up often in JEE, so let's begin.

0:34Sodium and other active metals give hydrogen

So, first the reaction with sodium. Two ROH plus two N a gives two RON a, plus H two. ROH is any alcohol: R is an alkyl group, and O H is the hydroxyl group. N a is sodium, the arrow means gives, and the small two written low in H two means two hydrogen atoms. Other active metals react the same way. Okay? Now compare the two sides of the equation. Which bond of the alcohol has broken here? The O H bond, yes, exactly! The little dash between O and H stands for that bond. Its hydrogen is replaced by sodium, giving RON a, a sodium alkoxide. Right?

1:18How strong an acid is an alcohol?

So an alcohol can lose the hydrogen of its O H group. Now let me ask you this: water, or ethanol, which one is the stronger acid? It is water, very good! H two O is the stronger acid; after it come primary, secondary and tertiary alcohols, in that order. The small circle written high means degree, so one degree is primary, and the greater than sign means a stronger acid than. With me? But why do alkyl groups weaken the acid? Alkyl groups release electrons onto the oxygen. Losing the hydrogen gives the alkoxide ion, R O minus; the small minus written high is a negative charge. Extra electron density on that oxygen destabilises this ion. Got it? Now put an alkoxide ion in water. R O minus plus H two O gives ROH plus O H minus: it takes a proton from water! So alkoxide ions are stronger bases than hydroxide. The weaker the acid, the stronger its conjugate base. Clear? Now, a classic trap. Why use sodium metal at all? Why not mix ethanol with aqueous N a O H, sodium hydroxide solution, to get sodium ethoxide? Will that work? No, that will not work. Ethoxide is the stronger base, so any ethoxide formed takes a proton back from water. Alcohols do not react with aqueous N a O H to any useful extent, and do not turn litmus red. So our puzzle is solved: ethanol is an acid, just a weaker one than water. Okay?

2:59The alcohol as a nucleophile: esterification

Now, the other side of an alcohol. Its oxygen has lone pairs of electrons, so an alcohol is also a nucleophile, and a weak base: strong acids protonate it. Right? As a nucleophile, it reacts with a carboxylic acid. RCOOH plus R prime O H gives RCOOR prime, an ester, plus water. The prime, the small dash on R, marks a second, possibly different, alkyl group. The pair of half arrows means the reaction is reversible. And we add a little concentrated sulphuric acid, H two S O four. With me? And since the reaction is reversible, we take the water out as it forms. That drives the equilibrium forward, towards the ester.

3:48Esterification: which bonds break?

Clear? Now, a common trap. Many students think the alcohol loses its whole O H group, and the acid loses only a hydrogen. Is that right? Which bonds really break? No, no, it is the other way round! The alcohol breaks its O H bond, so it loses only its hydrogen. The alcohol keeps its oxygen. Okay? And the acid breaks its carbon to O H bond, C dash O H on screen. So the acid gives the O H, the alcohol gives the hydrogen, and together they make water. Got it? Last, which alcohols form esters most easily? Primary, then secondary, then tertiary. The reason is steric: crowding round the oxygen slows its attack on the acid. Right?

4:36Acid chlorides and anhydrides: complete

So, how do we get the ester completely? Use an acid chloride or an acid anhydride instead of the acid. Then the ester forms completely. Okay? And with an acid chloride, we also add a base called pyridine. Now tell me, why add a base here? The base removes the H C l, hydrogen chloride, formed in the reaction. That shifts the equilibrium to the right, towards the ester. With me? Now, one more name. Putting COCH three in place of the H of an O H group is called acetylation; the acetylation of salicylic acid gives aspirin. Keep it in mind for Example two. Clear?

5:19Example 1: litres of H2 from 4.6 g ethanol with sodium

Now let's try Example one. Find the volume of hydrogen at STP, standard temperature and pressure, liberated when four point six grams of ethanol reacts completely with sodium. Pause the video and try it first. Done? Okay, let's check. For ethanol, two C two H five O H plus two N a gives two C two H five O N a, sodium ethoxide, plus H two. With me? Ethanol's molar mass, capital M, is forty six grams per mole, written g mol to the power minus one. Carbon gives twenty four, hydrogen six, and oxygen sixteen. So four point six over forty six gives zero point one mole of ethanol. Okay? So, how many moles of hydrogen do we get? Zero point one mole as well, right? Oh no, careful, look at the equation again. Two moles of ethanol give only one mole of H two. So zero point one mole of ethanol gives zero point zero five mole of hydrogen. Right? At STP, one mole of gas occupies twenty two point four litres; the cross means times, and capital L means litres. So zero point zero five times twenty two point four gives one point one two litres of hydrogen, our answer.

6:40Example 2: counting OH groups from the acetylation mass gain

Now a harder one. An alcohol of molar mass ninety two grams per mole is completely acetylated with ethanoic anhydride, and the product has molar mass two hundred and eighteen grams per mole. How many O H groups does it contain? Here is a hint: what does acetylation add? Pause the video and try it first. Done? Okay, let's check. In acetylation, each O H becomes OCOCH three. We lose the H, of mass one, and gain COCH three, of mass forty three. The numbers in brackets are these masses. Clear? So, each O H group adds forty three to the molar mass, right? Oh no, careful, we also lost a hydrogen! So the gain for each O H group is forty three minus one, which is forty two. Here is a topper's shortcut: every acetylated O H group adds forty two. Got it? The total gain is two hundred and eighteen minus ninety two, which is one hundred and twenty six. And one hundred and twenty six over forty two gives three. So there are three O H groups, and the alcohol is glycerol. See how quick that was?

7:58Quick revision: acidity, esterification, acid chlorides

So, quick revision. Water is a stronger acid than primary, secondary and tertiary alcohols, and alkoxides are made with sodium, not aqueous sodium hydroxide. Esterification is reversible; it breaks the O H bond of the alcohol and the carbon to O H bond of the acid. Acid chlorides and anhydrides give the ester completely, and pyridine removes the hydrogen chloride. Now go and try the practice problems on this page. All the best!