S p2 Hybridization

Ethene (C₂H₄)

π-bonding – bonding created by the side overlap of not hybrid p-orbitals in the plane, that goes through atoms’ nuclei. This plane makes it impossible for atoms to rotate free around the axis of bonding.

Cyklohexane (С₆H₁₀)

Differences between the σ bond and the p bond in the carbon-carbon double bond:

σ bond	p bond
Head-on overlap of the sp2 hydridized orbitals of two carbon atoms	Side-way overlap of the vacant p orbitals of two carbon atoms
The bonding electrons in are localized symmetrically along the internuclear axis of two bonded carbon atoms.	The electrons in the p bond appear as two lobes, one above and one below the internuclear axis of the two bonded atoms
Stronger	Weaker
Free to rotate	Restricted to rotation

sp Hybridization

E thyne (С₂H₂)

Ability to Catenate

Carbon atoms can link themselves together to form chains of varying length, branched chains and rings of different sizes.

Catenation: The ability of atoms in forming stable bonds with itself and joining up in chains or rings.

e.g.

3. Methods of representation of organic molecules

Molecular (empirical) formula shows qualitative and quantitative composition of molecule. It doesn’t show the structure.

Structural (graphical) formula shows qualitative and quantitative composition , and the order and the type of bonding of atoms in molecule.

Carbon has the valency 4 in all the organic compounds.

Kekulé structure – all bonds are shown as dashes, all atoms as symbols

isobutane acetone

• Condensed structure – some bonds are not shown, it shows only those bonds, which are necessary

or (СН₃)₂СНСН₃ or СН₃СОСН₃

• Skeletal structure – carbon skeleton is shown only by valence bonds, the atoms of carbon are not shown. Skeletal structure are used mostly in cyclocompounds:

cyclopropane aniline furane

4. A cross-coupling of atoms in a molecule

The commutual influence of atoms in molecule is defined by the kind of dividing of electron density in molecule and by the ability of covalent bonds to polarization.

Electron displacements are divided into two kinds: inductive and resonance (mesomeric) effects.

Inductive Effect

Due to the difference in electronegativity between two atoms linked up by s bonds, the bonding electrons will displace towards the more electronegative atom. The atom exhibits a partial negative charge.

The electronic effect of a group that is transmitted by the polarization of electrons in s bonds is called an inductive effect.

Inductive effect is represented by an arrow head in the middle of the covalent bond pointing in the direction of the displacement of electrons.

Electron-withdrawing group exerts a negative inductive effect.

Electron-donating group exerts a positive inductive effect.

1. Groups which exert negative inductive effects

(i.e. electron-withdrawing groups):

e.g.

–NO₂ > –F > –COOH > –Cl > –Br > –I

2. Groups which exert positive inductive effects (i.e. electron-releasing groups):

e.g.

alkyl groups like –CH₃, –C₂H₅, –C₃H₇

The main characteristics of inductive effect

• Always present when there are atoms with different electronegativity.

• Spreads only through σ-bonds and in only one direction.

• Dies out after 3-4 σ-bonds through the chain.

Conjugation

Conjugation (mesomerism) is the process of traffic of substituent electron influence through the conjugated system of π-bonds.

Conjugate system is the system that consists of simple and multiple bonds that alternates, or when there is an atom with a lone р-electron pair, or an atom that has an empty р-orbital, near the carbon atom that creates multiple bond. Conjugate systems can have open or closed chain.

Depended on the kind of overlapped orbitals, there can be different kinds of conjugation

Conjugation is energetically advantageous process that goes with energy emission. Conjugate systems has high thermodynamic stability.

Resonance effect is displacement of electron density in conjugate system

The displacement of electron density present in conjugate system only if there are electron-withdrawing groups or electron-releasing substituents.

Resonance (mesomeric) effect is marked as letter М.

Positive resonance effect is defined by substituents that gives electrons to the conjugate system.

Among them there are the atoms with negative charge (anions) or atoms that have a lone electron pair.

Negative resonance effect is defined by substituents that takes electrons from the conjugated system.

Among them there are atoms with empty orbital (carbocations) and atomic groups –СН=Х, or -С≡Х, where Х – more electronegative atom, than atom C.

The main examples of resonance effect:

The main characteristics of resonance effect

• It is present only if there is a conjugate system.

• It is present only if the conjugated system consists electron-withdrawing groups or electron-releasing substituents.

• It makes more displacement of electron density than inductive effect and particularly doesn’t die out through the chain

The action of both inductive and resonance effects

Inductive and resonance effects of the same substituent can have not the same direction.

The stronger influence on the polarization of molecules makes the resonance effect, because π-bonds can be polarized easier than σ-bonds. Halogens are the only exception.

Hyperconjugation is a stabilization effect resulting from interaction between C=C π-bond orbital and a C-H σ bond orbital on a neighboring substituent (σ,π-conjugation), or the empty orbital and a C-H σ bond orbital on a neighboring substituent (σ,р- conjugation).

T he more atoms of hydrogen are near the atom of carbon that is bonded with unsaturated system or with empty orbital, the bigger hyperconjugation effect is.

Further increasing the degree of substitution leads to still further stability of double bond

Acids and Bases

Acids and Bases: The BrØnsted Definition

Accoding to the BrØnsted definition, an acid is any species that donates a proton, and base is a substance that accepts a proton.

In a general sense

where Ka acidity constant, (in the dilute aqueous solution)

A stronger acid (larger K_a) has a smaller pK_a, and weaker acid (smaller K_a) has a larger pK_a

Relative Strengths of Same Common Acid and Their Conjugate Bases

	Acid	Name	pKa	Conjugate base	Name
Weaker acid Stronger acid	CH3CH2OH H2O HCN CH3COOH HF	Ethanol Water Hydrocyanic acid Acetic acid Hydrofluoric acid	16.00 15.74 9.31 4.76 3.45	CH3CH2O- HO- CN- CH3COO- F-	E thoxide ion Hydroxide ion Cyanid ion Acetat ion Fluorid ion	Stronger base Weaker base

Acids and Bases: The Lewis Definition

Of even more use is the Lewis definition of acid and base, which is not limited to compounds that gain or lose proton.

Lewis acid is a substance that accepts an electron pair and a

Lewis base is a substance that donates an electron pair

Questions and problems

1. On which orbital is situated electron that has the following quantum numbers n=3, l=0?

а)2s; b) 3d; c) 3s; d) 2p;

2. Which of the following formulas are the short structural formulas? а) ; b) СН₃СН₂ОН; c) ; d) С₂Н₆О; e) .

3. Which of the following compounds has no atoms that have lone electron pairs on the outside energetic level?

а) С_зОСН₃; b) СН₃NHСН₃; c) С₆Н₁₄; d) С₂Н₆.

4. The result of hybridization of one s- and three р-orbitals. а) σ-bond; b) sp-hybridization; c) sp²- hybridization; d) π-bond; e) sp³- hybridization.

5. What kind of hybridization has the atoms of carbon in the following compound

а) 1- sp³-, 2- sp²-, 3- sp²-, 4- sp²-, 5- sp²-,6- sp²-,7- sp²-;

b) 1- sp³-, 2- sp-, 3- sp-, 4- sp³-, 5- sp³-,6- sp³-,7- sp³-;

c) 1- sp³-, 2- sp³-, 3- sp³-, 4- sp³-, 5- sp³-,6- sp²-,7- sp²-;

d) 1- s p³-, 2- sp-, 3- sp-, 4- sp³-, 5- sp²-,6- sp²-,7- sp³-;

e) 1- s p-, 2- sp-, 3- sp³-, 4- sp³-, 5- sp³-,6- sp²-,7- sp²-.

6. What kind of conjugation is there in the following compound? Using the arrows of inductive and resonance effects show the situation of electron density in molecula а) π-π- conjugation; b) р-π- conjugation; c) σ-π- conjugation; d) σ-р- conjugation; e) р-р- conjugation; f) no conjugation

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