Naming, Structure and Physical Properties

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. When phenol is nitrated, two products form together, ortho nitrophenol and para nitrophenol. Same formula, each with an O H group and a nitro group. Now pass steam through the mixture. The ortho isomer is carried away with the steam, but the para isomer stays behind! Same atoms, so why such different behaviour? Today we name phenols, look at their structure, and see what decides their physical properties. These ideas come up often in JEE, so let's begin.

0:36What is a phenol?

First, the definition. Phenols have a hydroxyl group, the O H, attached directly to a carbon of a benzene ring. Keep that word, directly, in mind. Okay? The simplest one, C six H five O H, is phenol itself; the small numbers written low are subscripts. In round brackets is its other name, carbolic acid. Clear?

0:59Common names and IUPAC names

Now, names: the course table of common names and their IUPAC names. First, the cresols. The italic letters o, m and p stand for ortho, meta and para, and the numbers are positions on the ring, counted from the O H carbon. So ortho, meta and para cresol are two, three and four methylphenol. With me? Next, two O H groups on the ring. Catechol is benzene one two diol, and resorcinol is benzene one three diol. Right? Now you try. In hydroquinone, the two O H groups are para to each other, opposite across the ring. So what is its IUPAC name? Benzene one four diol, very good! Quinol, in round brackets, is another common name for it. Okay? Picric acid is two four six trinitrophenol, with three nitro groups at carbons two, four and six. Despite the word acid, its IUPAC name ends in phenol. Got it? And last, salicylic acid is two hydroxybenzoic acid, and salicylaldehyde is two hydroxybenzaldehyde. In these two, the O H appears as the prefix hydroxy, and carbon one is the ring carbon carrying the acid or the aldehyde group. Clear?

2:11Structure: a special C–O bond

Now, the structure. The carbon carrying the O H group is s p two hybridised; the small raised two is a superscript. And one lone pair of the oxygen is delocalised into the ring. With me? So let me ask you this. Compare the carbon oxygen bond, written C dash O, in phenol and in methanol. Is the bond in phenol shorter or longer? Shorter, yes, exactly! In phenol it is one hundred and thirty six picometres, written p m, and in methanol it is one hundred and forty two; the sign between them is less than. That delocalised lone pair gives the bond partial double bond character. Okay? Now a trap. Phenol has an O H group, just like an alcohol. So Lucas reagent should break its C O bond the same way, right? Oh no, careful, it does not! The C O bond of phenol is not broken by H X, by P C l five, which is phosphorus pentachloride, or by Lucas reagent, the way an alcohol's C O bond is. Here H X stands for the hydrogen halides, and the small five is a subscript. Clear?

3:15Phenol: physical properties

Now, physical properties. Phenol is a colourless solid. Its m p, the melting point, is three hundred and fourteen kelvin, written with a capital K, and its b p, the boiling point, is four hundred and fifty five kelvin. And it turns pink in air. Got it? Now bring in toluene. Capital M, in round brackets, is the molar mass. Toluene, with M equal to ninety two, boils at three hundred and eighty four kelvin; phenol, with M equal to ninety four, at four hundred and fifty five kelvin. Almost the same mass, so why are they seventy one kelvin apart? Yes, exactly, the O H group! Phenol forms intermolecular hydrogen bonds, bonds between different molecules, which must break before it boils. So phenol boils far higher than toluene, and the same O H group makes it fairly soluble in water. Right?

4:06Nitrophenols: inside or between molecules?

Now, back to our puzzle. In ortho nitrophenol, the O H group and the nitro group, N O two, sit side by side on the ring; in para nitrophenol, they are opposite each other. So which one can form a hydrogen bond inside its own molecule? The ortho one, exactly! Its O H and nitro groups are close enough to form an intramolecular hydrogen bond, one inside the same molecule. In round brackets is the name for this, chelation. Okay? Now, careful here. You might think any hydrogen bond raises the boiling point. But wait, this one is inside a single molecule, so it does not hold different molecules together. So ortho nitrophenol is steam volatile, lower boiling, and less soluble in water. Got it? Para nitrophenol, with its groups on opposite sides, cannot do this; it forms intermolecular hydrogen bonds and is not steam volatile. And that solves our puzzle: steam carries away the ortho isomer and leaves the para isomer behind. With me?

5:06Example 1: phenol or alcohol? Cresol, benzyl alcohol, more

Alright, Example one, an easy one from the course. Which of these are phenols: benzyl alcohol, para cresol, cyclohexanol, resorcinol and two phenylethanol? Remember that word, directly, and pause the video to try it first. Done? Okay, let's check. The rule is simple: the O H must sit on a carbon of the benzene ring. Para cresol and resorcinol have it there, so both are phenols; the double arrow means so. Right? Now benzyl alcohol. It has a benzene ring, and it has an O H group. So it is a phenol too, right? Oh no, careful, that is the trap! In benzyl alcohol, the O H sits on a side chain carbon, outside the ring, so it is an alcohol. Two phenylethanol is the same story. Okay? Cyclohexanol is a different trap. Its O H is on a ring carbon, but that ring is not a benzene ring, so it is an alcohol as well. Clear? So only two of the five are phenols: para cresol and resorcinol. Got it?

6:11Example 2: higher boiling point in three phenol pairs

Now Example two, a little harder. Which has the higher boiling point in each pair? Part A, ortho nitrophenol or para nitrophenol. Part B, phenol or anisole. And part C, catechol or hydroquinone. Use our puzzle, and pause the video to try it first. Done? Okay, let's check. Part A is our puzzle again; H bond on screen is short for hydrogen bond. The ortho isomer has an intramolecular one, the para isomer has intermolecular ones, so para nitrophenol boils higher. With me? In part B, anisole is C six H five OCH three, an ether. With no O H group, its molecules cannot hydrogen bond to each other, but phenol molecules can, so phenol boils higher. Right? Now part C, the tricky one. Catechol has its two O H groups at carbons one and two, side by side; hydroquinone has them at one and four, opposite each other. So which one boils higher? Hydroquinone, very good! Like ortho nitrophenol, catechol has some intramolecular hydrogen bonding, since its O H groups are neighbours. That means less bonding between molecules, so hydroquinone boils higher. Okay? So the answers are para nitrophenol, phenol and hydroquinone. Notice the pattern in parts A and C: groups side by side bond inside one molecule, and that lowers the boiling point. Clear?

7:38Quick revision: phenol names, C-O bond, hydrogen bonding

So, quick revision. Phenol has its O H directly on a benzene ring carbon; learn the name table by heart. Its C O bond is special: an s p two carbon, one hundred and thirty six picometres, and partial double bond character, so H X and Lucas reagent do not break it. Intermolecular hydrogen bonds raise the boiling point, while an intramolecular one, as in ortho nitrophenol, makes the compound steam volatile. Now go and try the practice problems on this page. All the best!