Comparing acidity and basicity, Lecture notes of Chemistry

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Comparing acidity and basicity itis very common to be asked to compare the acidity/basicity of two compounds, A and B. This guide teaches you how determine and explain which is more acidic and basic. Comparing acidity ® Molecules are usually those with -OH and COOH groups. © Benzene: 1. The benzene group is electron withdrawing. 2. [phenol only] Lone pair on O atom is delocalised onto the ring. 3. Conjugate base produced is more stable 4,Ais astronger acid than B because A has a benzene ring. ® Alkyl groups: 1. Alkyl group is electron releasing. 2. [alcohol only] Electrons are released towards the O atom. 3. The O atom attracts H+ more strongly4. A has more/longer alkyl groups attached to the C (in C-OH). Thus, electron releasing effect is stronger, making it a weaker acid. Note: no. of alkyl groups is more important than length. ® Carboxylic acids (COOH): 1. p-orbitals of C and O atoms in COOH overlap to form a delocalised system. 2. Conjugate base formed is more stable. 3. It loses H+ more easily, making it a stronger acid. 4. Ais more acidic than B because it has a COOH group. ® Halogens: 1. Halogen atoms are electronegative and draw electrons towards themselves. 2. Negative charge is drawn away from the O atom/COO- group. 3. Conjugate base formed is more stable. 4, It loses H+ more easily, making it a stronger acid. oS. Ais more acidic than B because it has more halogen atoms/more electronegative halogen. ® Priority when assessing acidity: COOH > benzene > alkyl/halogen Comparing basicity ® Molecules are usually those with amine (NH2) or amide (CONH2) groups. e Benzene: 1. The benzene group is electron withdrawing. 2. Lone pair on N atom is delocalised onto the ring. 3. N atom is less electronegative and less able to donate a lone pair to H+ ions. 4. Ais a weaker base than B because it has a benzene ~Y ring. — ® Alkyl group: 1. Alkyl group is electron releasing. 2. Electrons are released towards the N atom. 3. N atom is more electronegative and more able to donate a lone pair to H+ ions. 4.Ais astronger base than B because it has more/longer alkyl groups. ® Halogen: 1. The halogen atom is electron withdrawing. 2. Electrons are drawn away from the N atom. 35. The N atom is less electronegative and less able to donate a lone pair to H+ ions. 4. Ais a weaker base than B as it has more halogen atoms/more electronegative halogens. ® Amide: 1. Electrons on N atoms are delocalised because p- orbitals of C, N and O overlap. 2. Electron density on N atom decreases. 3. The N atom is less electronegative and less able to donate a lone pair to H+ ions. 4.4 is aweaker base than B because it is an amide. ® Amines are usually stronger bases than amides. However, when comparing different amines or amides, alkyl group > benzene/halogen