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Organic Chemistry – Some Basic Principles and Techniques - Structural Representations of Organic Compounds

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Complete Structural Formula: In this representation, every single covalent bond is shown as a dash (−-). It illustrates every atom and its connectivity within the molecule, such as in ethane (H−C(H)2−C(H)2−HH-C(H)_2-C(H)_2-H).

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Condensed Structural Formula: This notation simplifies the representation by omitting some or all of the dashes representing covalent bonds. Identical groups attached to an atom are often indicated by a subscript, for example, CH3CH2CH2CH2CH3CH_3CH_2CH_2CH_2CH_3 can be written as CH3(CH2)3CH3CH_3(CH_2)_3CH_3.

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Bond-Line Representation: This is a highly simplified method where carbon and hydrogen atoms are not explicitly shown. Lines represent C−CC-C bonds in a zig-zag fashion. Vertices and line ends represent carbon atoms. The number of hydrogens is inferred by the tetravalency of carbon (each carbon must have 44 bonds). Heteroatoms like OO, NN, or ClCl are always written.

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Three-Dimensional (3D) Representation: To represent 3D structures on a 2D surface, 'Wedge-Dash' notation is used. A solid wedge (▲\blacktriangle) represents a bond projecting out of the plane towards the viewer. A dashed wedge (line of parallel dashes) represents a bond pointing away from the viewer. Normal lines represent bonds in the plane of the paper.

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Tetravalency of Carbon: Carbon always forms four covalent bonds in organic compounds to achieve a stable octet, which is the basis for calculating implicit hydrogens in bond-line structures.

📐Formulae

CnH2n+2 (General formula for Alkanes)C_nH_{2n+2} \text{ (General formula for Alkanes)}

CnH2n (General formula for Alkenes)C_nH_{2n} \text{ (General formula for Alkenes)}

CnH2n−2 (General formula for Alkynes)C_nH_{2n-2} \text{ (General formula for Alkynes)}

Degree of Unsaturation (DU)=C+1−H+X−N2\text{Degree of Unsaturation (DU)} = C + 1 - \frac{H + X - N}{2}

💡Examples

Problem 1:

Expand the following bond-line formula into a complete structural formula: A zig-zag line with 4 vertices where the second vertex has a vertical line pointing up and the third vertex has an 'OHOH' group attached.

Solution:

The structure is CH3−CH(CH3)−CH(OH)−CH3CH_3-CH(CH_3)-CH(OH)-CH_3.

Explanation:

In bond-line notation, every end and corner is a Carbon. The chain has 44 carbons. Carbon 22 has a methyl group (indicated by the branch), and Carbon 33 has a hydroxyl group. Hydrogens are added to satisfy the 44 bonds required for each Carbon.

Problem 2:

Write the condensed formula and the bond-line formula for 2,2,42,2,4-Trimethylpentane.

Solution:

Condensed formula: (CH3)3CCH2CH(CH3)2(CH_3)_3CCH_2CH(CH_3)_2. Bond-line: A zig-zag line of 55 carbons (pentane) with two strokes on Carbon 22 and one stroke on Carbon 44.

Explanation:

The parent chain is pentane (55 carbons). At position 22, there are two methyl groups, and at position 44, there is one methyl group. In the condensed form, (CH3)3C(CH_3)_3C represents a quaternary carbon.

Problem 3:

Identify the number of Carbon and Hydrogen atoms in a simple regular hexagon bond-line structure (Cyclohexane).

Solution:

C6H12C_6H_{12}

Explanation:

A regular hexagon in bond-line notation represents a cyclic ring of 66 carbon atoms. Each vertex (carbon) is connected to 22 other carbons. To satisfy the tetravalency of 44, each carbon must be bonded to 22 hydrogen atoms. Thus, 6×2=126 \times 2 = 12 hydrogens.