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Questions and Answers
What is the bond angle characteristic of sp3 hybridization?
What is the bond angle characteristic of sp3 hybridization?
Which hybridization results in a linear geometry?
Which hybridization results in a linear geometry?
What is the order of bond strength from weakest to strongest?
What is the order of bond strength from weakest to strongest?
What effect does a +I group have on carbocations?
What effect does a +I group have on carbocations?
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Which of the following bond lengths is the shortest?
Which of the following bond lengths is the shortest?
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The inductive effect operates through which type of bonds?
The inductive effect operates through which type of bonds?
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Which of the following groups has the strongest -I effect?
Which of the following groups has the strongest -I effect?
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What is the % of S character in sp2 hybridization?
What is the % of S character in sp2 hybridization?
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Which of the following carbocations is the most stable?
Which of the following carbocations is the most stable?
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Which acid is stronger based on their pKa values?
Which acid is stronger based on their pKa values?
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What is a key requirement for hyperconjugation to occur?
What is a key requirement for hyperconjugation to occur?
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Which of the following correctly describes the stability of alkenes?
Which of the following correctly describes the stability of alkenes?
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Which of the following carbocations has the least stability?
Which of the following carbocations has the least stability?
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What type of stability does hyperconjugation provide to alkyl free radicals?
What type of stability does hyperconjugation provide to alkyl free radicals?
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In terms of acidity, which of the following compounds has the highest pKa?
In terms of acidity, which of the following compounds has the highest pKa?
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What is called when a compound can be represented by more than one structural form?
What is called when a compound can be represented by more than one structural form?
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What does resonance energy measure?
What does resonance energy measure?
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What is the +R or +M effect associated with?
What is the +R or +M effect associated with?
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Which statement accurately describes steric inhibition of resonance?
Which statement accurately describes steric inhibition of resonance?
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How do intermolecular hydrogen bonds affect boiling points?
How do intermolecular hydrogen bonds affect boiling points?
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In which situation would a compound be less soluble in a solvent?
In which situation would a compound be less soluble in a solvent?
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What characterizes aromaticity in a molecule?
What characterizes aromaticity in a molecule?
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What is intramolecular hydrogen bonding?
What is intramolecular hydrogen bonding?
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Which of the following effects can hinder resonance?
Which of the following effects can hinder resonance?
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Study Notes
Hybridization
- Hybridization is the mixing of atomic orbitals with similar energies to form new hybrid orbitals.
- Hybrid orbitals have identical shapes and equivalent energies.
SP3 Hybridization
- One s and three p orbitals hybridize, forming four identical sp3 hybrid orbitals.
- Each sp3 orbital contains one unpaired electron.
- This results in a tetrahedral geometry with a bond angle of 109.28°.
SP2 Hybridization
- One s and two p orbitals hybridize, forming three sp2 hybrid orbitals.
- This results in a trigonal planar geometry with a bond angle of 120°.
SP Hybridization
- One s and one p orbital hybridize, forming two sp hybrid orbitals.
- This results in a linear geometry with a bond angle of 180°.
Bond Properties
- Bond length is inversely proportional to the percentage of s character.
- Bond strength is proportional to the percentage of s character and electronegativity.
Bond Length
- C-C bond length: 1.54 Å
- C=C bond length: 1.34 Å
- C≡C bond length: 1.20 Å
Bond Strengths and Bond Energies
- Sp3 < sp2 < sp (Bond strength order)
Inductive Effect/Transmission Effect
- Inductive effect is the electron shift along a chain of atoms due to the presence of a polar covalent bond.
- This is a permanent effect.
-I Effect
- Electron-withdrawing inductive effect occurs when an atom or group attracts the electron pair towards itself.
+I Effect
- Electron-donating inductive effect occurs when an atom or group pushes electrons away from itself.
Decreasing Order of -I Effect
- NO2 > CN > F > COOH > Cl > Br > I > OCH3 > C6H5
Decreasing Order of +I Effect
- (CH3)3C > (CH3)2CH > C2H5 > CH3
Application of Inductive Effect
Stability of Carbocations
- +I groups increase the stability of carbocations.
- Increasing order of stability: (CH3)3C+ < (CH3)2CH+ < CH3CH2+ < CH3+
Relative Acid Strength of Formic Acid and Acetic Acid
- The methyl group has a +I effect, decreasing the proton release from the O-H group in acetic acid.
- Therefore, acetic acid is weaker than formic acid.
- pKa of formic acid = 4.76
- pKa of acetic acid = 3.77
Relative Acid Strengths
- Decreasing order of acid strength: F2CH2COOH > ClCH2COOH > BrCH2COOH > ICH2COOH
- pKa of F2CH2COOH = 2.57
- pKa of ClCH2COOH = 2.86
- pKa of BrCH2COOH = 2.90
- pKa of ICH2COOH = 3.16
Hyperconjugation (No Bond Resonance)
- Alkyl substituents with hydrogens on a carbon-carbon double bond act as electron donors to the pi system.
- Hyperconjugation is the electronic interaction between the C-H sigma bond and the empty p-orbital of the adjacent sp² hybridized carbon.
- This is also known as the Baker-Nathan effect.
Structural Requirements of Hyperconjugation
- The compound must have at least one sp² hybridized carbon.
- The alpha carbon with respect to the sp² hybridized carbon must have at least one hydrogen atom.
Types of Hyperconjugation
6(C-H), π conjugation
- Occurs in alkenes and alkyl substituted aromatic compounds.
6(C-H) positive charge vacant p-orbital conjugation
- Occurs in alkyl carbocations.
6(C-H) odd electron conjugation
- Occurs in alkyl free radicals.
Application of Hyperconjugation
Stability of Carbocations
- Stability ∝ number of alpha hydrogen atoms + 1.
- Decreasing order of stability: (CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+
Stability of Alkenes
- Stability ∝ number of hyperconjugation structures.
- Decreasing order of stability: (CH3)2C=CH2 > CH3CH=CH2 > CH2= CH2
Stability of Alkyl Free Radicals
- Stability ∝ number of hyperconjugation structures.
- Decreasing order of stability: (CH3)3C• > (CH3)2CH• > CH3CH2• > CH3•
Resonance
- Resonance is the phenomenon where a compound can be represented by multiple contributing structures.
- These structures are called resonating structures or canonical structures.
- The more stable the resonance structures, the more stable the compound.
Resonance Energy
- The resonance hybrid is more stable than any of the contributing structures.
- Resonance energy is the difference in energy between the actual structure and the most stable hypothetical structure.
Resonance Effect/Mesomeric Effect
- Resonance effect is the delocalization of π electron systems through conjugated systems.
Types of Resonance Effect
+R or +M effect
- Electron-donating effect where an atom or group releases electrons towards the conjugated π system.
-R or -M effect
- Electron-withdrawing effect where an atom or group withdraws electron density from the conjugated systems through resonance.
Steric Inhibition of Resonance
- Resonance requires overlapping orbitals to be coplanar.
- Bulky groups at the ortho position can hinder this coplanarity, leading to steric inhibition of resonance (ortho effect).
Hydrogen Bonding
- Hydrogen bonding involves the attraction between a hydrogen atom and a highly electronegative atom (e.g., F, O, N).
- Strongest hydrogen bonds involve O, N, F, or other small atoms.
Types of Hydrogen Bonding
Intermolecular Hydrogen bond
- Formed between atoms of different molecules.
Intramolecular Hydrogen bond
- Formed between atoms within the same molecule.
Effect of Hydrogen Bond on Physical Properties
Effect on Melting Point and Boiling Point
- Intermolecular hydrogen bonds increase melting and boiling points.
- Intramolecular hydrogen bonds have a negative effect on melting and boiling points.
Effect on Solubility
- Compounds forming intermolecular hydrogen bonds with a solvent are soluble in that solvent.
- Compounds hindering hydrogen bonding with the solvent are less soluble.
Aromaticity
- Aromaticity is the unusual stability of cyclic molecules such as benzene, despite the presence of double bonds.
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