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Orbitals – s, p – different shapes

Ways of explaining stability of organic molecules. Orbitals – s, p – different shapes. Valence Bond model – hybridisation, s and p bonds. Molecules with p bonds. Molecules with s bonds. Resonance (2.5,2.6) Stability if more forms. Electronegativity (2.1) Inductive effects.

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Orbitals – s, p – different shapes

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  1. Ways of explaining stability of organic molecules Orbitals – s, p – different shapes Valence Bond model – hybridisation, s and p bonds Molecules with p bonds Molecules with sbonds Resonance (2.5,2.6) Stability if more forms Electronegativity (2.1) Inductive effects Conjugation (14.1) Alternate p bonds Aromaticity (15.3) Alternate p bonds and 4n+2 electrons Hyperconjugation (6.6) p bonds & C-H bonds

  2. Covalent bonds can have ionic character These are polar covalent bonds Bonding electrons attracted more strongly by one atom than by the other Electron distribution between atoms is not symmetrical Polar Covalent Bonds: Electronegativity Inductive effect: shifting of electrons in a bond in response to EN of nearby atoms

  3. Acids and Bases: The Brønsted–Lowry Definition • A Brønsted acid is a substance that donates a hydrogen ion (H+) • A Brønsted base is a substance that accepts the H+ • “proton” is a synonym for H+

  4. Ka – the Acidity Constant • Ka ranges from 1015 for the strongest acids to very small values (10-60) for the weakest • pKa = -log Ka • A smaller value of pKa indicates a stronger acid and is proportional to the energy difference between products and reactants

  5. How to predict if a compound will be a strong/weak acid? If A- (RHS) is more stable – then HA is strong acid How can A- be stabilised relative to HA? 1. Negative charge must end up on electronegative atom Electronegative – likes electrons (negative charge) Usually O-H more acidic than N-H > C-H – carboxylic acid, alcohol more acidic than acetone)

  6. If A- (RHS) is more stable – then HA is strong acid How can A- be stabilised relative to HA? 2. Inductive effects Other electronegative atoms – draw negative charge away, spread out Compounds with CF3 more acidic than CH3 page 604, chapter 17 – section 17.2 - alcohols page 759, chapter 20 - section 20.4 – carboxylic acids

  7. If A- (RHS) is more stable – then HA is strong acid How can A- be stabilised relative to HA? 3. A- has many ‘good’ resonance forms relative to HA Many resonance forms – spread/delocalisation of negative charge eg COOH compared with COH, or alkyl-OH and phenol Carboxylic acid vs alcohol Section 20.2 page 757 Alkyl alcohol vs phenol Section 17.2, page 605-606 How many resonance forms can be drawn? How many resonance forms can be drawn?

  8. If A- (RHS) is more stable – then HA is strong acid How can A- be stabilised relative to HA? (less common factors) 4. A- has aromatic character, HA does not HA not aromatic, but A- aromatic – e.g. cyclopentadiene see page 526 - chapter 15, section 15.4 Cyclopentadiene gives away H+ so that it can achieve aromatic stability Cyclooctatriene?

  9. If A- (RHS) is more stable – then HA is strong acid How can A- be stabilised? (less common factors) 5. A- - negative charge ends up on orbitals with more s character (e.g. sp) Alkynes more acidic than alkenes>alkanes page 271 – chapter 8, section 8.7 – read up yourself How about dibasic acids? How about bases?

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