Chemistry General introduction , Tetravalence of carbon: Shapes of organic compounds, Structural representations of organic compounds

### Topics to be covered

=> General introduction
=> Tetravalence of carbon: Shapes of organic compounds
=> Structural representations of organic compounds

### GENERAL INTRODUCTION

• Organic compounds are vital for sustaining life on earth and include complex molecules like genetic information bearing deoxyribonucleic acid (DNA) and proteins that constitute essential compounds of our blood, muscles and skin.

• Organic chemicals appear in materials like clothing, fuels, polymers, dyes and medicines.

color{red}("HISTORICAL DEVELOPMENT OF ORGANIC CHEMISTRY")

• Berzilius, a Swedish chemist proposed that a ‘vital force’ was responsible for the formation of organic compounds. However, this notion was rejected in 1828 when F. Wohler synthesised an organic compound, urea from an inorganic compound, ammonium cyanate.

color{red}(underset("Ammonium cyanate")(NH_4CNO) overset("Heat")→ underset("Urea")(NH_2CONH_2))

• The pioneering synthesis of acetic acid by Kolbe (1845) and that of methane by Berthelot (1856) showed conclusively that organic compounds could be synthesised from inorganic sources in a laboratory.

• The development of electronic theory of covalent bonding ushered organic chemistry into its modern shape.

### TETRAVALENCE OF CARBON: SHAPES OF ORGANIC COMPOUNDS

star color{green}("The Shapes of Carbon Compounds")

Hybridisation influences the bond length and bond enthalpy (strength) in organic compounds.

The color{red}(sp) hybrid orbital contains more s character and hence it is closer to its nucleus and forms shorter and stronger bonds than the color{red}(sp^3) hybrid orbital.

The color{red}(sp^2) hybrid orbital is intermediate in color{red}(s) character between color{red}(sp) and color{red}(sp^3) and, hence, the length and enthalpy of the bonds it forms, are also intermediate between them.

The greater the s character of the hybrid orbitals, the greater is the electronegativity. Thus, a carbon atom having an color{red}(sp) hybrid orbital with color{red}(50% s) character is more electronegative than that possessing color{red}(sp^2) or color{red}(sp^3) hybridised orbitals.

star color{green}("Some Characteristic Features of π Bonds")

In a color{red}(π)(pi) bond formation, parallel orientation of the two color{red}(p) orbitals on adjacent atoms is necessary for a proper sideways overlap. Thus, in color{red}(H_2C=CH_2) molecule all the atoms must be in the same plane.

The color{red}(p) orbitalare mutually parallel and both the color{red}(p) orbitals are perpendicular to the plane of the molecule. Rotation of one color{red}(CH_2) fragment with respect to other interferes with maximum overlap of color{red}(p) orbitals and, therefore, such rotation about carbon-carbon double bond color{red}((C=C)) is restricted.

The electron charge cloud of the color{red}(π) bond is located above and below the plane of bonding atoms. This results in the electrons being easily available to the attacking reagents(color{red}(π) bonds provide the most reactive centres in the molecules containing multiple bonds).

Q 3224223151

How many σ and π bonds are present in each of the following molecules?

(a) HC≡C CH=CHCH_3
(b) CH_2=C=CHCH_3

Solution:

(a) σ_(C – C) : 4; σ_(C–H) : 6; π_(C=C) : 1; π C≡C : 2
(b) σ_(C – C) : 3; σ_(C–H) : 6; π_(C=C) : 2.
Q 3244223153

What is the type of hybridisation of each carbon in the following compounds?

(a) CH_3Cl,
(b) (CH_3)_2CO,
(c) CH_3CN,
(d) HCONH_2,
(e) CH_3CH=CHCN

Solution:

(a) sp^3,
(b) sp^3, sp^2,
(c) sp^3, sp,
(d) sp^2,
(e) sp^3, sp^2, sp^2, sp
Q 3264223155

Write the state of hybridisation of carbon in the following compounds and shapes of each of the molecules.

(a) H_2C=O,
(b) CH_3F,
(c) HC≡N.

Solution:

(a) sp^2 hybridised carbon, trigonal planar;
(b) sp^3 hybridised carbon, tetrahedral;
(c) sp hybridised carbon, linear.

### STRUCTURAL REPRESENTATIONS OF ORGANIC COMPOUNDS

star color{green}("Complete, Condensed ") color{green}("and Bond-line Structural Formulas")

The Lewis structure or dot structure, dash structure, condensed structure and bond line structural formulas are some of the specific types. The Lewis structures, however, can be simplified by representing the two-electron covalent bond by a dash (-).A single dash represents a single bond, double dash is used for double bond and a triple dash represents triple bond. Lone pairs of electrons on heteroatoms (e.g., oxygen, nitrogen, sulphur, halogens etc.) may or may not be shown. Thus, ethane color{red}((C_2H_6)), ethene color{red}((C_2H_4)), ethyne color{red}((C_2H_2)) and methanol color{red}((CH_3OH)) can be represented by the following structural formulas. Such structural representations are called color{red}("complete structural formulas.")

We can also use color{red}("condensed structural formula") by omitting some or all of the dashes representing covalent bonds and by indicating the number of identical groups attached to an atom by a subscript.

In bond-line structural representation of organic compounds, carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are drawn in a zig-zag fashion. The only atoms specifically written are oxygen, chlorine, nitrogen etc. The terminals denote methyl color{red}((–CH_3)) groups (unless indicated otherwise by a functional group), while the line junctions denote carbon atoms bonded to appropriate number of hydrogens required to satisfy the valency of the carbon atoms. Some of the examples are represented as follows:
(i) 3-Methyloctane can be represented in various forms as:

(ii) Various ways of representing 2-bromo butane are:

In cyclic compounds, the bond-line formulas may be given as follows:

star color{green}("Three-Dimensional Representation of Organic Molecules")

star The three-dimensional color{red}((3-D)) structure of organic molecules can be represented on paper by using certain conventions. For example, by using solid and dashed wedge formula, the color{red}(3-D) image of a molecule from a two-dimensional picture can be perceived.
star Solid-wedge is used to indicate a bond projecting out of the plane of paper, towards the observer. The dashed-wedge is used to depict the bond projecting out of the plane of the paper and away from the observer.

Q 3214223159

Expand each of the following condensed formulas into their complete structural formulas.
(a) CH_3CH_2COCH_2CH_3
(b) CH_3CH=CH(CH_2)_3CH_3

Solution:

Q 3224323251

For each of the following compounds, write a condensed formula and also their bond-line formula.

(a) HOCH_2CH_2CH_2CH(CH_3)CH(CH_3)CH_3

(b) N equiv C - overset(overset(OH)(|))CH-C equiv N

Solution:

Condensed formula:
(a) HO(CH_2)_3CH(CH_3)CH(CH_3)_2
(b) HOCH(CN)_2

Bond-line formula:

Q 3244323253

Expand each of the following bond-line formulas to show all the atoms including carbon and hydrogen

Solution: