물리유기화학

면담시간: 매주화오후 2-4시 (과학관 507호)참고교재: Modern Physical Organic Chemistry

E. V. Anslyn and D. A. Dougherty성적평가: 시험 2번, 수업태도포함홈페이지: chem.yonsei.ac.kr/~injae전화: 02-2123-2631E-mail: injae@yonsei.ac.kr

Introduction to Structure Models of bondingChapter 1

Chemical bonding -> structure, stability and reactivity

1.1 A review of basic bonding conceptsSolutions of Schrödinger equation: wavefunctions -> atomic or molecular orbitals

Principal quantum number: n = 1, 2, 3,..Magnetic quantum number: p orbital; -1, 0, 1 (px, py, pz),

d orbital; -2, -1, 0, 1, 2 (dxy, dxz, dyz, dz2, dx2-dy2)Spin quantum number: +1/2, -1/2

Electron configuration: C, 1s2 2s2 2p2 -> excitation; 1s2 2s1 2p3

1s

carbon

Valence number: the number of bonds that an atom can make (e.g. C; 4, N; 3, O; 2, H; 1)

VSEPR (valence-shell electron-pair repulsion rule: all groups emanating from an atom will be spatial positions that are as far apart from another as possible.

Two models for considering chemical bonding in organic molecules

- Valence bond theory (VBT): most atoms of the binding energy between the atoms at the most stable internuclear separation results from exchange (resonance) of electrons between the two nuclei. -> electrons are localized between specific atoms in a molecule.

- Molecular orbital theory (MOT): electrons are distributed among a set of molecular orbitals of discrete energies which can extend over the entire molecules.

A B A B A B

1 2 2 1+

Valence bond theory (VBT)

The key ideas that are used to adapt the concepts of VBT to complex molecules are hybridization and resonance.

Hybridization: the method of adding and substrating atomic orbitals on the same atom.

C: 2s 2px 2py 2pz-> 4sp3 4 equivalent bonds

The highly directional sp3 orbitals providefor more effective overlap and stronger bonds

The second concept: resonance, an extension of VBT which recognizes more than one Lewis structure can be written.

-> describe electron delocalization

-> is particularly useful in describing conjugated systems and reactive intermediates

Polarity: important property of chemical bondsInternuclear electrons in a covalent bond will be subject to a probability distribution that favors one of the two atoms.

Electronegativity: the tendency of an atom to attract electrons (originally developed by Linus Pauling)

Electronegativity: hybridization effects

As s-character of carbons increases, electronegativity of carbon increases

sp > sp2 > sp3 (≡C > =C > –C)50% 33% 25%

pKa: ≡C-H, =C-H, –C-H ≡C-, =C-, –C-

~24 ~44 ~50

S (핵근처에전자가많음)

P (핵근처에전자가적음)

more stable

Inductive effects: - successive polarization through bonds

- the phenomenon of withdrawing electrons through bonds to the more electronegative atom or group.

Field effects:

- a polarization in a molecule that results from charges that interact through space,

rather than through bonds

Bond diploes

When two atoms of differing electronegativity are bonded, one end og the bond will be δ+

and the other will be δ-. This analysis leads to the notion of a bond dipole as the local

moment that is associated with a polar covalent bond.

A dipole moment (μ) provides a means of comparing which bonds are more polar.

μ = q x r

+q -qr Unit: Debye (1D = 3.336 x 10-30 coulomb·m,

두전하±e 가 0.2082 Å떨어져있을때의 dipole moment)

Molecular dipole momentsIs a well-defined, intrinsic property of a molecule. A molecule has a dipole moment whenever the center of positive charge in the molecule is not conincident with the center of negative charge.

Polarizability

Another property that is closely related to electronegativity and position in the Periodic Table is polarizability.The ability of the electron cloud to distort in response to an external field is known as its polarizability. Upon distortion, a dipole is typically induced in the molecule, adding toany permanent dipole already present.

1. Move left to right across a row of the periodic table, polarizability decreases.C > N > O > F, CH4 > NH3 > H2O

2. Move down a column in the periodic table, polarizability increases substantially.S > O, P > N, H2S > H2O

참고

Polarizability – hardness and softness

Softness: the ease of distortion of electron cloudHardness: the difficulty of distortion of electron cloud

Principle of hard and soft acids and bases (HSAB)- Hard acids (electrophiles) form stronger (and reacted faster) with hard bases (nucleophiles) - Soft acids (electrophiles) form stronger (and reacted faster) with soft bases (nucleophiles)

hard-likes-hard -> electrostatic interactionssoft-likes-soft -> covalent bond formation

1. Highly electronegative atoms -> harder2. Larger atoms are softer than smaller ones with similar electronegativity3. Metal cations become harder as the oxidation number increases.

참고

참고

HO + H-OH2 faster than HO + Br-Br

CH2 = CH2 + Br-Br faster than CH2=CH2 + H-OH2

Hard base Hard acidHard acid Hard base soft base

soft nucleophile soft nucleophile

soft nucleophile hard

Eg 1)

Eg 2)

Eg 3)

KCN + Et-I Et-CN

AgCN + Et-I Et-N=C:isocyanide

cyanide (nitrile)

:N C: :N C:..harder one soft one

:N C: I Et-CN

:C N: I Ag+δ+δ

carbocation -> hard electrophile

Et-N=C:

Bond lengths: 분자에따라가장변하지않는요소

1.2 A more modern theory of organic bonding – Molecular orbital theory

MO theory considers the electrons in molecules to occupy MOs that are formed by linear combinations (addition and subtraction) of all the atomic orbitals on all the atoms in the structure.

In MOT, electrons are not confined to an individual atom plus the bonding region with another atom. Instead, electrons are contained in MOs that are highly delocalized – spread across the entire molecule.

MOT is based on the Schrödinger equation.

HΨ = EΨ

H: Hamiltonian operator

Ψ: wavefunction describing an orbital

E: the energy of an electron in a particular orbital

-> obtain Ψ and this equation

Ψ = Σciφi (linear combinations of all the atomic orbitals)

ci = coefficient

φi = atomic orbital

Qualitative molecular orbital theory (QMOT)

Rules1. Consider valence orbitals only2. Form completely delocalized MOs as linear combinations of s and p AOs.3. MOs must be either symmetric or antisymmetric with respect to the symmetry operations

of the molecule.4. Compose MOs for structures of high symmetry and then produce orbitals for related

but less symmetric structures by systematic distortions of the orbitals for highest symmetry5. Molecules with similar molecular structures, such as CH3 and NH3, have qualitatively

similar MOs, the major difference being the number of valence electrons that occupy the common MO system.

6. The total E = sum of the MO energies of individual valence electrons.7. If the two highest energy MOs of given symmetry derive primarily from different kinds of

AOs, then mix the two MOs to form hybrid orbital.8. When two orbitals interact, the lower orbital is stabilized and the higher energy orbital is

destabilized

9. When two orbitals interact, the lower energy orbital mixes into itself the higher energy onein a bonding way, while the higher energy orbital mixes into itself the lower energy one in an antibonding way.

10. The smaller the initial energy gap between two interacting orbitals, the stronger the mixing interaction.

11. The larger the overlap between interacting orbitals, the larger the interactions.12. The more electronegative elements have lower energy AOs. 13. A change in the geometry of a molecule will produce a large change in the energy

of a particular MO if the geometry change results in changes in AO overlap that are large.14. The AO coefficients are large in high energy MOs with many nodes or complicated nodal

surfaces.15. Energies of orbitals of the same symmetry classification can not cross each other.

Instead, such orbitals mix and diverse.

Example

C-C bondbond energy: ~83 kcal/mol

C-O bondbond energy: ~85.5 kcal/mol

CC pp

σ bond

σ* bond

O

Cp

p

σ bond

σ* bond

Eσ Ec Eo

Eσ > Eo but 2Eσ < Ec + Eo

C-C: homolytic cleavageC-O: heterolytic cleavage

C=C bond C=O bond

CC pp

π bond

π* bond

O

Cp

p

π bond

π* bond

Eπ Ec Eo

C-C: homolytic cleavageC-O: heterolytic cleavage

C O

C O

N C: OCN

O

N C: CH2No reactions

Cl3C

Cl3C O No reactions

Cl3C

1.4 Bonding and structures of reactive intermediatesCarbocations: a structure with a positive charge that is associated with a carbon center

Carbenium ions: trivalent species with a formula of R3C+

Carbonium ions: - pentavalent species of the general formula R5C+ (not common in the solution

but common in the gas phase)- carbocations that have an important contribution from three center-two electron bonding.

Carbenium ions: trivalent species with a formula of R3C+

+ +

planar pyramidal- more stable -less stable

Stability3o > 2o > 1o > methyl cation

Hyperconjugation (VBT): no-bond resonance structure

Why?

Carbenium ions are especially prone to rearrangement.

H

HH

RR

H

RR

HH

sp2

Carbonium ions: - pentavalent species of the general formula R5C+ (not common in the solution

but common in the gas phase)- carbocations that have an important contribution from three center-two

electron bonding.

CH5+

Carbanions: a structure with a negative charge that is associated with a carbon center, R3C:-

pyramidal

sp3

In most cases the barrier to inversion at a carbanion center is small, roughly 1-2 kcal/mol (inversion barrier) for simple systems

Several factors can significantly raise the inversion barrier at a carbanion center.1. Cyclopropyl anions

109.5o 120o

High inversion barrier

2. Electronegative substituents attached to the anionic center can also substantiallyraise the inversion barrier.

Electronegative substituents preferentially stabilize the sp3 ground state over the sp2

transition state, raising the inversion barrier.

NH3: ~5 kcal/mol, NF3: ~50 kcal/mol

3. Substituents that stabilize a carbanion by π delocalization will favor the planar structure.

참조: NH3 : ~5 kcal/mol, PH3 : 35 kcal/mol

sp2sp3

Radicals: methyl radicals has seven valence electrons, H3C.

No obvious preference for planar or pyrimidal geometry can be predicted.Simple radicals show only a very weak preference for the planar structure, and simple substitution can produce pyramidal radicals.

Factors that favor pyrimidalization in radicals.- an electronic effect of the sort discussed previously for anions. CF3

. is very strongly pyramidal. Electronegative substituents prefer bonding to an sp3

hybrid rather than an sp2.

CarbenesR2C:

triplet state singlet state

Triplet states should be preferred at the linear geometry (Hund’s rule), and indeed it is.H-C-H angle becomes small enough -> singlet statesAngle is 136o for the triplet and 105o for the singlet.

While simple carbenes have a triplet ground state, approapriate substituents can reverse this preference.-> carbenes with lone-pair donating substituents such as N, O, and halogens

can have singlet ground states because of such an interactions