Organic Chemistry - Hybridization and Bond Properties (PDF)

# Hybridization and Bond Properties ## Hybridization - The mixing of atomic orbitals having nearly the same energies and the redistribution of their energies to form new orbitals is called **hybridization**. - These new orbitals are called **hybrid orbitals** which have identical shapes and equival...

# Hybridization and Bond Properties ## Hybridization - The mixing of atomic orbitals having nearly the same energies and the redistribution of their energies to form new orbitals is called **hybridization**. - These new orbitals are called **hybrid orbitals** which have identical shapes and equivalent energies. ### SP3 Hybridization * All four orbitals (one *s* and three *p*) are hybridized to produce four identical hybrid orbitals. * Each *sp3* orbital has one unpaired electron. * This results in a **tetrahedral** geometry with a **bond angle** of 109.28° ### SP2 Hybridization * One *s* and two *p* orbitals of the valence shell of the central atom of the given molecule combine to form 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 of the valence shell of the central atom of the given molecule combine to form two *sp* hybrid orbitals. * This results in a **linear** geometry with a **bond angle** of 180°. ## Bond Properties ### Bond Angles and Geometry of Molecules | Type of HB | Bond Angle | Geometry | % of S Character | |-------------|-------------|-------------|-------------------| | SP | 180° | Linear | 50 | | SP2 | 120° | Trigonal Planar | 33.33 | | SP3 | 109.5° | Tetrahedral | 25| ### Bond Length * Bond length ∝ 1/ % of S Character * **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 * Bond strength ∝ 1/ Bond length ∝ % of S Character ∝ Electronegativity * *sp3* < *sp2* < *sp* (Bond strength order) ## Inductive Effect/Transmission Effect - This is a permanent effect operating in polar covalent bonds. - The process of electron shift along a chain of atoms due to the presence of a polar covalent bond is known as **inductive effect or transmission effect**. ### -I Effect - When the atom or group attracts the electron pair to itself, it is known as *-I effect* or **electron withdrawing inductive effect**. ### +I Effect - When the atom or group pushes the electrons away from itself, it is known as *+I effect*. **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 group** increases 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 an electron releasing inductive effect (+I effect) which decreases the proton release from the O-H group. - 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 (having hydrogen on a carbon) on a carbon-carbon double bond act as electron donors to the pi system. - This electronic interaction is known as **no-bond resonance or hyperconjugation**. - This is due to the 6-π or 6-P (incomplete or vacant) orbital overlap, also known as the Baker-Nathan effect. ### Structural Requirements of Hyperconjugation 1. The compound should have at least one sp² hybrid carbon. 2. The alpha carbon with respect to the sp² hybrid carbon should have at least one hydrogen atom. ### Types of Hyperconjugation - **6(C-H), π conjugation:** This type of conjugation occurs in alkenes and alkyl substituted aromatic compounds. - **6(C-H) positive charge vacant p-orbital conjugation:** This type of conjugation occurs in alkyl carbocations. - **6(C-H) odd electron conjugation:** This type of conjugation occurs in alkyl free radical. ## Application of Hyperconjugation ### Stability of Carbocations - Stability of Carbocations ∝ number of alpha hydrogen atoms + 1. - **Decreasing order of stability:** (CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+ ### Stability of Alkenes - Stability of alkenes ∝ number of hyperconjugation structures. - **Decreasing order of stability:** (CH3)2C=CH2 > CH3CH=CH2 > CH2= CH2 ### Stability of Alkyl Free Radical - Stability of Alkyl Free Radical ∝ number of hyperconjugation structures. - **Decreasing order of stability:** (CH3)3C• > (CH3)2CH• > CH3CH2• > CH3• ## Resonance - When a compound is written in more than one form, this phenomenon is called **resonance**. - The contributing structures are called **resonating structures or canonical structures**. - The more stable a resonance structure, the more stable the compound. ### Resonance Energy - The resonance hybrid of a species is a more stable structure than any one of the resonating structures contributing to it. - The **resonance energy** is a measure of the extra stability of the resonance hybrid. - It is defined as the difference in energy between the actual structure of the species and the most stable of the hypothetical structures (i.e. canonical structures). ## Resonance Effect/Mesomeric Effect - One part of the π electron system can be delocialised from one part of the system to other parts of the system (having conjugated π system) due to the phenomena of resonance. - This is called **resonance effect or mesomeric effect**. ### Types of Resonance Effect - **+R or +M effect:** If any atom or group releases electron/donates electrons towards the conjugated π system, due to this resonance, this effect is known as **+R effect or +M effect**. - **-R or -M effect:** If any atom or group withdraws electron density through conjugated systems due to resonance, this effect is called **-R or -M effect**. ## Steric Inhibition of Resonance - For resonance, overlapping orbitals must be coplanar. - However, the presence of bulky groups at the ortho position to the resonance will not occur. - This is known as **steric inhibition of resonance or ortho effect**. ## Hydrogen Bonding - Hydrogen bonding refers to the kind of bond in which hydrogen is linked to a highly electronegative atom such as fluorine by electrostatic forces of attraction. - The strongest hydrogen bonds are those involving O,N,F, or other atoms having small nuclei. ### Types of Hydrogen Bonding - **Intermolecular hydrogen bond:** It is formed between atoms of two or more molecules resulting in the association of molecules. - **Intramolecular hydrogen bond:** It is formed between two atoms within the same molecule. ## Effect of Hydrogen Bond on Physical Properties ### Effect on Melting Point and Boiling Point - Compounds containing intermolecular hydrogen bonds have higher melting and boiling points compared to non-hydrogen bonded hydrocarbons. - Intramolecular hydrogen bond has a negative effect on melting and boiling point. ### Effect on Solubility - Compounds which can form intermolecular hydrogen bonds with a solvent will be soluble in that solvent. - Compounds in which hydrogen bonding with the solvent is hindered will be less soluble in that solvent. ## Aromaticity - Benzene is a remarkable molecule because it exhibits unusual stability despite the presence of double bonds. - This unusual stability is known as **aromaticity**. ### Aromatic Compounds - Aromatic compounds follow the 4n+2 π electron rule or **Hückel’s rule**. - This rule was proposed by Erich Hückel, a German physicist, in the early 1930s. ### Conditions For Aromaticity - The molecule must be: - **Cyclic** - **Planar** - Have a continuous loop of overlapping p-orbitals. - Have a total of (4n + 2) π electrons, where n = 0, 1, 2, 3,.... ## Antiaromatic Compounds - These compounds are cyclic, planar, and conjugated. - They have 4n π electrons. - They are usually unstable and reactive. ## Non-Aromatic Compounds - These compounds do not meet all the requirements for aromaticity. - They can be: - Non-cyclic - Non-planar - Not have a continuous loop of overlapping p-orbitals - Have a total of (4n) π electrons.