Electronegativity, pKa, and the Inductive Effect

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Last updated 9:28 AM on 1/14/26
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35 Terms

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Definition of Electronegativity

A chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) towards itself

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Electronegativity is determined by what?

  • Nuclear charge (more protons = more pull)

  • Number and location of electrons in atomic shells

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How does electronegativity change across a periodic table?

Increases from left to right of the periodic table, and from bottom to top

  • F is the most electronegative

<p>Increases from left to right of the periodic table, and from bottom to top</p><ul><li><p>F is the most electronegative</p></li></ul><p></p>
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What explains the reduction in electronegativity when you descend a group?

There is extra shielding of the shells of electrons

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What effect does electronegativity have on charge distribution of a molecule?

When there are difference in electronegativity, atoms will become slightly positive (δ+) and slightly negative (δ-)

This leads to induction and inductive effect

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The stronger the electronegativity….

….the stronger the inductive effect

<p>….the stronger the inductive effect</p>
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What is the inductive effect?

The permanent shifting of electron density along σ-bonds due to electronegativity differences.

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What is a -I group?

An electron-withdrawing group that pulls electron density away through σ-bonds

X =

  • Br

  • Cl

  • NO2

  • OH

  • SH

  • SR

  • NH2

  • NHR

  • NR2

  • CN

  • COOH

  • CHO

  • C(O)R

<p>An electron-withdrawing group that pulls electron density away through σ-bonds</p><p>X =</p><ul><li><p>Br</p></li><li><p>Cl</p></li><li><p>NO<sub>2</sub></p></li><li><p>OH</p></li><li><p>SH</p></li><li><p>SR</p></li><li><p>NH<sub>2</sub></p></li><li><p>NHR</p></li><li><p>NR<sub>2</sub></p></li><li><p>CN</p></li><li><p>COOH</p></li><li><p>CHO</p></li><li><p>C(O)R</p></li></ul><p></p>
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What is a +I group?

An electron-donating group that pushes electron density through σ-bonds (e.g. alkyl groups, metals).

X =

  • R (alkyl or aryl)

  • Metals (e.g. Li, Mg)

<p>An electron-donating group that pushes electron density through σ-bonds (e.g. alkyl groups, metals).</p><p>X =</p><ul><li><p>R (alkyl or aryl)</p></li><li><p>Metals (e.g. Li, Mg)</p></li></ul><p></p>
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How does distance affect the inductive effect?

The inductive effect decreases rapidly as distance from the substituent increases.

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What does pKa measure?

Acid strength.

Lower pKa = stronger acid = more willing to donate H⁺.

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How do electron-withdrawing groups affect pKa of acids?

They stabilise the conjugate base → lower the pKa → increase acidity.

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How do electron-donating groups (EDG) affect pKa of acids?

  • They destabilise the conjugate base

  • Raise the pKa

  • Decrease acidity.

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What’s the pKa of carboxylic acid?

≈ 5

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What’s the pKa of phenol?

≈ 10

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What’s the pKa of alcohol?

≈ 15

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Why does phenol have a lower pKa than alcohol?

Phenol is more acidic than alcohols because its conjugate base (phenoxide) is resonance stabilised, whereas alkoxides have a localised negative charge and are destabilised by +I alkyl groups.

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What is resonance?

The delocalisation of electrons over several atoms, represented by multiple canonical forms.

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Why does resonance stabilise molecules/ions?

Because charge is spread over a larger area, lowering energy and increasing stability.

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Why are benzene bonds all the same length?

  • Because π electrons are delocalised over the ring (true structure is a resonance hybrid).

<ul><li><p>Because π electrons are delocalised over the ring (true structure is a resonance hybrid).</p></li></ul><p></p>
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Carboxylate resonance forms

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Why are resonance forms useful?

  • Resonance forms are useful for showing electron movement and reactivity, even though none is fully correct.

  • Curly arrows are used to show the movement of bond electrons between resonance forms.

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Definition of the Mesomeric effect

Electron donation or withdrawal through resonance involving π systems and lone pairs.

  • Double bond adjacent to a lone pair of electrons

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+M

When electron density is pushed into the π-bond (orbital).

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-M

Where a π-orbital overlaps with an adjacent p-orbital that is low in electron density.

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<p>How do +M groups affect phenol acidity?</p>

How do +M groups affect phenol acidity?

They destabilise the phenoxide ion → increase pKa → make phenol less acidic.

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How do –M groups affect phenol acidity?

They stabilise the phenoxide ion → decrease pKa → make phenol more acidic.

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What does pKa mean for bases?

It refers to the protonated form of the base; higher pKa = stronger base.

<p>It refers to the protonated form of the base; higher pKa = stronger base.</p><p></p>
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What happens when pH = pKa for a base?

About 50% of the base is protonated.

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Why are alkyl amines more basic than ammonia?

Alkyl groups donate electron density (+I), making nitrogen more able to accept H⁺.

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<p>Why is phenylamine much less basic than cyclohexylamine?</p>

Why is phenylamine much less basic than cyclohexylamine?

The lone pair in phenylamine is delocalised into the aromatic ring, reducing availability to bind H⁺.

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<p>Why is pyrrole very weakly basic?</p>

Why is pyrrole very weakly basic?

Its lone pair is part of the aromatic system and cannot accept a proton.

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Why are amides extremely weak bases?

The lone pair on nitrogen is delocalised into the carbonyl group by resonance.

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Why are carbonyl groups polar?

Oxygen is more electronegative than carbon, giving O δ⁻ and C δ⁺.

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How do substituents affect carbonyl reactivity?

  • Electron-withdrawing groups increase δ⁺ on carbon → more reactive

  • Electron-donating groups decrease δ⁺ → less reactive