Organic Chemistry Module 5 Prof Steve Glover — Riggs Building

Topic 5A Introduction

SOOT DIAMOND GRAPHITE BUCKY BALLS Carbon

DNA C + N itrogen C + N itrogen O xygen C + N itrogen O xygen P hosphorous Carbon the basis of life

Organic chemistry Organic chemistry is the chemistry of CARBON containing compounds Berzelius — 1808: "Organic" substances as they were known could not be synthesised in the laboratory from "inorganic" materials, because it was believed that a "vital force" which was possessed only by living organisms was necessary to produce organic compounds. 2

Organic chemistry August Kekulé — 1830: Organic chemistry is the chemistry of CARBON containing compounds. Leopold Gmelin 1848: “Organic chemistry is the chemistry of carbon containing compounds which occur naturally” Not quite but fairly close to the truth. 2

Studying organic chemistry Learn the language of organic chemistry— how to name and identify organic chemicals Be able to draw the structures of organic chemicals Learn the various categories of organic chemicals: –Learn what are their properties –Learn what are their characteristic reactions Understand how and why organic chemicals react the way they do 2

Why is carbon special Carbon — carbon and carbon — hydrogen covalent bonds are extremely strong ( C—C bond energy of 350kJmol -1 ). Carbon can form single, double and triple bonds readily. Can form a huge variety of chains and rings (by joining ends of chains). 3

Covalent bonds Covalent bonds are formed by the sharing of a pair of electrons between two atoms with little or no differences in electronegativity (at least smaller than 1.9) Each atom attains a closed shell electronic configuration (satisfies the octet rule) for at least some of the time Covalent bonds are depicted by lines between bonded atoms, eg: 3

Multiple bonds Carbon can form more than one covalent bond with a neighbour, in which case we draw several lines between the atom Triple bonds eg HCN: Double bonds eg ethylene: 3

Carbon containing compounds Over10 million compounds listed in Chemical Abstracts and most are organic. Chains and rings can be formed: 3

Tetrahedral carbon We note that carbon is tetravalent in all cases, ie forms four bonds to other atoms. The carbons are not flat. The Lewis structures represent carbon at the centre of a tetrahedron, eg methane: Bonding electron pairs repel one another least this way. 4

Lewis structures Carbon and hydrogen differ in electronegativity by 0.4 Carbon forms strong covalent bonds with itself and with hydrogen Example of simplest hydrocarbon — methane 4

Lewis structures Covalent bonds can be represented by Lewis formulas which show outer shell (valence) electrons around the atoms. Often only those electron pairs involved in forming the covalent bond are shown as a shorthand device. We simplify this to line-bond formulas where a single line represents each shared electron pair e.g. 4

VSEPR “Valence-shell electron repulsion” explains the geometry of a wide range of molecules: 120° 180° 120° 4

Polarity in organics Carbon bonded to electronegative atoms results in polar bonds. 5

Models of organics Density surfaces and electrostatic potential surfaces 6

Resonance in carboxyl groups 6 Lone pair on oxygen can “delocalise” onto the carbonyl oxygen Use double-headed arrows to show movement of electrons

A chemist’s shorthand 3-D representations 7

A chemist’s shorthand formulas — ethane (C 2 H 6 ) Empirical formula CH 3 Molecular formula C 2 H 6 Structural formula Condensed formula CH 3 —CH 3 or CH 3 CH 3 7

A chemist’s shorthand for pentane (C 5 H 12 ) Skeletal for CH 3 CH 2 CH 2 CH 2 CH 3 : Hydrogens omitted. They are understood to be there to complete the valencies. 7