Organic Chemistry 2: Alkanes and Alkenes Lecture 2023 PDF

Organic Chemistry 2: Alkanes and Alkenes Emdadul Haque, Ph.D. Department of Biochemistry & Molecular Biology Carbon Bonding in Alkanes The valence bonds or points of attachment may be represented in structural formulas by a corresponding number of dashes attached to each atom: C N O H Cl S...

Organic Chemistry 2: Alkanes and Alkenes Emdadul Haque, Ph.D. Department of Biochemistry & Molecular Biology Carbon Bonding in Alkanes The valence bonds or points of attachment may be represented in structural formulas by a corresponding number of dashes attached to each atom: C N O H Cl S F Br I The bonds in an alkane are symmetrically directed toward the corners of a tetrahedron. Tetrahedral structure of carbon: (a) a regular tetrahedron; (b) a carbon atom with tetrahedral bonds; (c) a carbon atom within a regular tetrahedron; (d) a methane molecule, CH4 Writing the Correct Structural Formula for Propane, C3H8 H H H H C C C H H H H An alkane contains only single bonds. It is possible to write two structural formulas corresponding to the molecular formula, C4H10 H H H C H C H C C H H H C H C C C H H H H H H H H H H butane or n-butane bp = -0.5 C mp = -138.3 C isobutane bp = -11.7 C mp = -159.5 C H Isomerism The phenomenon of two or more compounds having the same molecular formula but different structural arrangements of their atoms is called isomerism. The various individual compounds are called isomers. e.g. There are two isomers of C4H10 Figure 1.2 Ball-and-stick models illustrating structural formulas of methane, ethane and propane. Figure 1.2 Ball-and-stick models illustrating structural formulas of butane and isobutane. Isomers are compounds that have the same molecular formula, but different structural formulas. Example for Drawing Isomers There are three isomers of pentane, C5H12. Write structural formulas and condensed structural formulas for these isomers. 1. Write the carbon skeleton of the compound containing the longest continuous carbon chain. C-C-C-C-C 2. Complete the structure by attaching hydrogen atoms around each carbon. H H H H H H C C C C C H H H H H H CH3CH2CH2CH2CH3 Isomers of Pentane, C5H12 (continued) For the next isomer, start by writing a four-carbon chain and attach the fifth carbon atom to either of the middle carbon atoms – do not use the end ones. Add 12 hydrogens to complete the structure. H H H H H H C H C C C C H H H H CH3 H CH3CH2CHCH3 CH3CH2CH(CH3)2 This is the only Carbon four-chain compound that can be written. Isomers of Pentane, C5H12 (continued) For the third isomer, start with a three-carbon chain, attach the other carbon atoms to the central carbon atom, and complete the structure by adding 12 hydrogen atoms: H H H H H C H C C C H C H H H H CH3 H CH3CCH3 CH3 No more isomers of C5H12 can be written Naming Organic Compounds IUPAC System for Naming Organic Compounds • The method recommended by the International Union of Pure and Applied Chemistry is systematic, generally unambiguous, and internationally accepted. • Some trivial or common names are still used such as iso-butane. Alkyl Groups Alkyl groups have the general formula CnH2n+1 (one less hydrogen than the corresponding alkane). The name of the group is formed from the name of the corresponding alkane by simply substituting –ane by –yl. CH4 = methane -CH3 = methyl The letter R is often used in formulas to mean any of the many possible alkyl groups. Table 2.1 Names and Formulas of Selected Alkyl Groups Alkyl groups If there is methyl group located on the second carbon of a carbon chain, we can use the prefix iso-. The prefix "sec" or "s" is used when the functional group is bonded to a secondary carbon. The prefix "tert" or "t" is used when the functional group is bonded to a tertiary carbon. Primary (1°), Secondary (2°), and Tertiary (3°) Carbon Atoms primary CH3 primary primary H2 C CH H3C tertiary C H2 CH3 secondary IUPAC Rules for Naming Alkanes 1. Select the longest continuous chain of carbon atoms as the parent compound, and consider all alkyl groups attached to it as branch chains that have replaced hydrogen atoms of the parent hydrocarbon. The name of the alkane consists of the name of the parent compound prefixed by the names of the branch-chain alkyl groups attached to it. CH3 H2 C CH H3C C H2 CH3 C H2 The longest continuous chain is 6 carbons; the parent hydrocarbon is hexane. IUPAC Rules for Naming Alkanes 2. Number the carbon atoms in the parent carbon chain from one end to the other, starting at the end closest to the first carbon atom that has a branch chain. 3. Name each branch-chain alkyl group and designate its position on the parent carbon by a number. 2-methyl CH3 1 H3C CH 2 3 C H2 H2 C 4 2-methylhexane 5 CH3 C H2 6 IUPAC Rules for Naming Alkanes 4. When the same alkyl-group branch chain occurs more than once, indicate this by a prefix (di-, tri-, tetra-, etc.) written in front of the alkyl group name (e.g. dimethyl indicates two methyl groups). The numbers indicating the positions of these alkyl groups are separated by a comma, followed by a hyphen, and placed in front of the name. CH3 C 1 H3C 2 3 CH CH3 H2 C 4 5 CH3 C H2 6 CH3 2,2,3-trimethylhexane IUPAC Rules for Naming Alkanes 5. When several different alkyl groups are attached to the parent compound, list them in alphabetical order (e.g. ethyl before methyl as in 3-ethyl-4-methyloctane and 4-ethyl3-methyloctane). CH3 1 H3C H2 C 3 CH 2 CH 4 5 C H2 H2 C 6 7 CH2 C H2 8 CH2 H3C 3-ethyl-4-methyloctane If a first substituent occurs equally close to either terminus of the parent chain, the nearness of the second substituent to the termini of the chain is determinative (and so on, to the third or fourth substituent, if necessary). If two different chains of equal length are present, choose the one as parent which has the larger number of branches (substituents). Iso-propyl 3-isopropylhexane 3-ethyl-2-methylhexane Cycloalkanes Alkanes containing rings of carbon atoms are called cycloalkanes. Cycloalkanes again only contain carbon-hydrogen bonds and carboncarbon single bonds, but this time the carbon atoms are joined up in a ring. According to IUPAC rules, cycloalkanes are named by placing the prefix cyclo before the name of alkane with same number of carbon. The smallest cycloalkane is cyclopropane. Methylcyclohexanae 1 3 2 3 1,3-Dimethylcyclohexane 2 1 1-Ethyl-3methyl cyclopentane EXERCISE Name the following alkane: CH3 H2 C H3C CH CH CH2 H2C CH3 H2 C CH CH3 H2 C C H2 CH3 4-ethyl-5,6-dimethyldecane Write the structural formula for 3-ethylpentane Write the structural formula for 2,2,4-trimethylpentane What is the correct name of the following alkane? 6 7 5 2 4 2 1 3 3 4 5 6 7 1 2 6 1 3-ethyl-5-methylheptane ? 7 5-ethyl-3-methylheptane ? What is the name of the following alkane? What is the name of the following alkane? Write the name of the following compounds?