Comparing acidities of phenol, ethanol and ethanoic acid

CAPE Unit 2 Chemistry: Module 1 Chemistry of Carbon compounds

comparing acidities of alcohols, phenols and carboxylic acids

Refer to CAPE Unit 2 guide pages 52-53

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Introduction

Acidity is determined by (i) how easily the H+ ions can be lost from the compound (ii) the stability of the carbanion formed when the H+ ions are lost

• The acids we will be dealing with in this module would be carboxylic acids

• The acidity of carboxylic acids is determined by the conjugative effect and the inductive effect

• The conjugative effect occurs in molecules where there appears to be alternating single and double bonds but instead the orbitals overlap and form an extended delocalised system

• The inductive effect is the polarization of a bond due to electronegativity differences between the atoms. Click here for more.

Comparing the acidity of ethanoic acid and ethanol

Effect of chlorine substituentS on carboxylic acids

• In terms of the carbanion formed, because of the -I effect of the chlorine atom, that negative charge is also spread over the chlorine atom

• The greater the delocalization of the charge, the more energetically stable the carbanion will be and the less likely it will want to recombine with the H+

• The greater the number of Chlorine atoms, the greater the charge is spread leading to even more stabilty.

• Chloroethanoic acids are more acidic, lower pKa value, than ethanoic acid

• The negative inductive efffect of the highly electronegative chlorine atom draws electron density away from the C in the -COOH group.

• This pull/shift in electron density by Cl further enhances the pull/shift that is already happening in the O-H bond

• The O-H bond is now weakened even more and H+ is readily lost

• From Unit 1, we learned that acidity of weak acids can be expressed in terms of Ka or pKa values

• Recall that Ka is the dissociation constant for weak acids and pKa=-log Ka

• The larger the Ka value the more acidic the compound as the [products]>[reactants and H+ ions and the carbanion formed=products

• The smaller the pKa value, the more acidic the compound

Comparing acidities of ethanol, phenol and carboxylic acid

Let us look at ethanol

• Acidity is also explained by the stability of the carbanion formed. And the carbanion is rendered stable when there is DELOCALIZATION of the charge which inhibits recombination with the removed H+

• The negative charge on the ethoxide is concentrated or LOCALIZED on the oxygen atom and this facilitates easy recombination of the ethoxide ion and the removed H+

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• The pKa value is 16.0. This suggests it is an extremely poor acid

• However we can look at why it may be considered "acidic" at all.

• Acidity is defined by ability to release a proton (H+) so just the presence of the electronegative atom, Oxygen, attached to the hydrogen atom will facilitate bond breakage to release the H+ ion

What about phenol

Phenol has the -OH grroup directly attached to the benzene ring. Acidity is dependent on the stability of the phenoxide anion. In the phenoxide anion the negative charge is delocalized and becomes a part of the ring system so it is less available to attract the H+ ion.

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summary video comparing acidities of alcohol, phenol and carboxylic acids

THE DIFFERENCE IN ACIDITIES OF ETHANOL, PHENOL AND ETHANOIC ACID CAN BE OBSERVED IN CHEMICAL REACTIONS

All three react with Na metal to give off hydrogen gas Only phenol and ethanoic acid (carboxylic acids) are acidic enough to react with NaOH, not ethanol Only ethanoic acid, not ethanol or phenol. is acidic enough to react with carbonates and hydrogencarbonates

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SUMMARY

pKa=measure of acidity in weak acids. Small pKa=Large Ka= stronger (weak ) acid. The conjugative effect(mesomeric/resonance) effect and inductive effect have an impact on acidity.

Inductive effect

Conjugative effect

-I=negative inductive effect/electron withdrawing species e.g oxygen, chlorine etc=stronger acid +I=positive inductive effect=electron donating species e.g alkyl groups= weaker acid

Delocalization=increased stability of carbanion= stronger acid

June 2011 Unit 2 Qu 4(c)

Explain the difference in acidity of alcohols, phenols and carboxylic acids. In your response, make reference to acid strength, inductive effect and conjugative effects

Acid strength increases in the order : alcohol, phenol, carboxylic acid The acidity of organic compounds is determined by the stability of the carbanion formed in the dissociation reactions of each of the organic compounds. The more stable the carbanion, the stronger the acid

Ethanoic acid. Ethanoic acid forms the most stable carbanion, the ethanoate ion. Because of the conjugative effect, the electrons in the COO- become delocalised between the two oxygen atoms . This reduces the electron density on any one oxygen atom as the negative charge is spread. This conjugative effect stabilises the ethanoate ion. The conjugative effect is greater than in phenol, so the stabilisastion of the ion is greater

Phenol: Phenoxide ion is stablised as one of the lone pairs of electrons on the oxygen atom overlaps with the delocalised electrons in the benzene ring reducing the electron density on the oxygen atom . This forms an extended delocalized system increasing the conjugative effect and stabilising the phenoxide ion. Note that the conjugative effect is greater than the inductive effect so movement is toward the benzene ring than the electronegative oxygen.

Ethanol: No conjugative efffect exists to stabilise the ethoxide ion. The charge on the the ethoxide ion is concentrated on the oxygen atom because of the positive inductive effect of the ethyl group . This makes the ethoxide ion readily attract H+ ions to form back ethanol making it the least acidic of the three

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