ORGANIC CHEMISTRY [MH5; Chapter 22, Tutorial Notes] • The common feature of all organic compounds is that they contain the element carbon together with only a few other elements, principally hydrogen, oxygen and nitrogen. • The number of known organic compounds already exceeds ten million, a number vastly larger than that of all other elements taken together (with the exception of hydrogen), and the possible number is virtually limitless. • For this reason one modern definition of organic chemistry is the chemistry of carbon compounds. Features of Organic Compounds • Organic compounds are molecular, rather than ionic. • Each carbon atom always forms a total of 4 covalent bonds. • Carbon atoms may be bonded to each other, or to other non metal atoms; commonly hydrogen, a halogen, oxygen or nitrogen. – 379 – SHAPES OF ORGANIC MOLECULES • The arrangement of atoms bonded to a carbon atom follows VSEPR theory....... 3 CH4: four single bonds tetrahedral sp hybrids 2 C2H4: two single bonds trig. planar sp hybrids one double bond CO2: two double bonds linear sp hybrids C2H2: one single bond, linear sp hybrids one triple bond – 380 – • Carbon occurs as three allotropes, with three different structures. • The incredible hardness of diamond results from a three- dimensional network of C—C bonds; it is in fact a single giant molecule; each carbon is bonded to four other carbons. • In contrast, graphite is built up from two-dimensional sheets of sp2 hybridized C atoms which can easily slip and slide over one another. • The third allotrope of carbon exists as a network of 60 carbon atoms arranged in a sphere; this form is commonly known as a “buckyball”. • Organic molecules exhibit isomerism,both structural isomerism and stereoisomerism. • This means two distinct and different compounds can have the same molecular formula. – 381 – Structural Classification of Organic Compounds • Most organic compounds fall into a small number of groups. • Within each of these groups, all compounds have similar chemical and physical properties and can be synthesized by similar reactions. • So....... we can concentrate on learning the characteristics of these groups, or families without discussing individual compounds in detail. • These "groups" or homologous series can be classified according to similarities in structure in two basic ways. • The first is the Skeletal classification. •The backbone of all organic compounds (except for those having only one C atom) is a skeleton of C atoms linked to each other in chains or rings. • These C chains or rings are quite stable; which means that they survive unchanged throughout most chemical reactions. • The majority of the remaining bonds to Carbon are satisfied by H atoms. • Compounds that contain only C and H, the hydrocarbons, are regarded as the parent structures. Hydrocarbons aliphatic aromatic acyclic cyclic alkanes alkenes alkynes alkanes alkenes alkynes • The second classification is that of Functional Groups. • All other compounds may be derived from the parent structures by replacement of one or more H atoms by other atoms or "groups of atoms". • It is the nature of these other atoms which determines the characteristic chemical properties or functionality of the compounds. THE SKELETAL CLASSIFICATION – 382 – • We begin with the hydrocarbons, compounds containing only C and H atoms. • The first subdivision is the aliphatic hydrocarbons, which may be divided into acyclic compounds (having no rings of C atoms) and cyclic compounds (having rings of C atoms). Acyclic alkanes [MH5; 22.1] • The distinctive feature of the acyclic category is that the C atoms are linked in chains only. • This basic skeletal structure may be characterized further, according to the presence or absence of either branches in the chain or of multiple bonds linking the C atoms. • A continuous sequence of C atoms in an unbranched chain has often been called a straight chain; not really correct in view of the 109.5E bond angles!! • When all of the C atoms are linked by single bonds only, the compound is said to be saturated, and is called an alkane. • The first eight members of the acyclic alkane family are: – 383 – • A particularly important characteristic of any such homologous series of compounds is that it can be represented by a general molecular formula. • The general formula for acyclic aliphatic hydrocarbons is CnH2n+2 (where n = 1,2,3.... the number of Carbon atoms). • Let’s take a look at butane, C4H10: • We see that TWO different arrangements of the atoms are possible, one with an unbranched chain, and the other with a branched chain. • Both have molecular formula C4H10 but different molecular structures; they are isomers, and the general phenomenon is termed isomerism. • Isomers are categorized as either structural isomers or stereoisomers. – 384 – ISOMERISM IN ALKANES Structural Isomerism • When the differences between isomers arise from a difference in which atoms are bonded to which other atoms, the isomers are termed structural isomers. • If one looks at compounds of C and H, it becomes apparent that three of the simplest compounds, CH4, C2H6 and C3H8, can have only one bonding arrangement. • Each C atom forms 4 bonds; each H atom 1 bond. – 385 – •For C4H10, we saw that two non-equivalent bonding arrangements are possible, depending upon how the C atoms are linked. A: CH3CH2CH2CH3..... m.p. !138E b.p. 1E density 0.579 g mLG1 B: CH3CH(CH3)2 ..... m.p. !159E b.p. !12E density 0.549 g mLG1 AB • Each isomer has its own unique physical and chemical properties, which can differ greatly. • For the two isomers of C4H10, the only way that isomer A could possibly be converted into isomer B would be if two single bonds were broken, CH3 and H were to exchange positions, and two new bonds were to form. (This does not happen under normal conditions.) • Because the energy required to break many single bonds is about 300!400 kJ molG1 at ordinary temperatures, structural isomers are normally stable and distinct chemical species. • When you are drawing structural isomers you must remember that one or more bonds must be broken to change one bonding arrangement into another. • This is characteristic of all structural isomers. – 386 – Drawing Structural Isomers C6H14 C4H9CR – 387 – • The number of structural isomers possible for large molecules can be enormous, as the following data show: CH4 none C6H14 5 C2H6 none C7H16 9 C3H8 none C8H18 18 C4H10 2C10H22 75 C5H12 3C20H42 366,319 Cyclic or Cyclo Alkanes, CnH2n • It is also possible for an unbranched chain of Carbon atoms to form a ring through loss of two H atoms, and formation of a C - C sigma (δ) bond. EXAMPLE: Hexane is C6H14: It can form a ring: • As a result, Cyclohexane, C6H12 has two less H atoms than C6H14. • The smallest number of carbon atoms that can form a ring is three, which results in cyclopropane, C3H6. – 388 – ORGANIC CHEMISTRY SHORTHAND • The most common method for depicting rings is by simple polygons, in which each corner is a C atom. • Hydrogen atoms are not shown; the number of H’s at each corner C is the difference between 4 and the number of line-bonds shown. • Also, each line bond is assumed to terminate in a C atom. • This is illustrated in the following example where the number of H atoms at each C (identified by number) is indicated: 8 3 2 7 4 1 5 6 There are at: C1 0 H atoms C2 1 H atoms C3,C4,C5 2 H atoms C6, C7, C8 3 H atoms – 389 – • Each type of ring system is classified also according to the number of rings (mono- vs poly-cyclic) and, more importantly, the degree of unsaturation (discussed later). CH2 CH2 or CH3 or CH2 CH2 C4H8 C6H12 Linked Ring Decalin Fused Rings Norborane Bridged Ring System • Polycyclic molecules can differ in the structural relationship between rings. • The rings may be separate and independent and simply linked together or they may share one or more atoms. •Thus fused ring systems share two adjacent atoms (e.g. decalin) and bridged ring systems share at least three atoms (e.g. norbornane). – 390 – Bond Rotation and Conformers • In addition to structural isomers, differing short-lived arrangements of atoms in a molecule, termed conformations, can result by rapid rotation of atoms, or groups of atoms, around single bonds. • When two atoms are connected by a single bond, the atoms are free to spin about the bond axis. • For simple molecules such as HCR, this rotation of the two atoms with respect to the bond axis has no effect on the molecular geometry, and has no structural consequences. • For a bond to persist, overlap between orbitals must be maintained. Rotation about the H —CR bond ( a σ-bond ) does NOT break the overlap. • For molecules containing more than two atoms, however, the rotation about a single bond axis may change the molecular geometry. • When a rotation around a single bond does result in a change in the molecular geometry, the structures which can be drawn are called conformations, and the molecules are conformers. – 391 – •The concept of a preferred conformation is simply illustrated using ethane, C2H6: A B C HH H H H H H H C C H C C C C H H H H H H H H H •Figure A shows ethane as a flat molecule (which we know is not so!!). •Figure B shows ethane in 3-D (recall the wedges indicate the bond coming forward and the dotted line indicate the bond going away). •Figure C shows a different conformation of ethane; one “end” of the molecule has rotated, so the positions of the Hydrogen atoms with respect to each other have changed. • There is another notation system known as Newman Projections; these show more effectively the radial distribution of the atoms attached to two adjacent atom centres.