Assistant Lecture Aayad Amaar

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1 Assistant Lecture Aayad Amaar An Introduction to : The Saturated Hydrocarbons Organic chemistry is the study of carbon-containing compounds. The bonds between carbon and another atom are almost always covalent bonds, whereas the bonds in many inorganic compounds are ionic bonds. Ionic bonds result from the transfer of one or more electrons from one atom to another. Thus, ionic bonds are electrostatic, resulting from the attraction between the positive and negative ions formed by the electron transfer. Covalent bonds are formed by sharing one or more pairs of electrons. The most general classification of organic compounds divides them into hydrocarbons and substituted hydrocarbons. A hydrocarbon molecule contains only carbon and hydrogen. A substituted hydrocarbon is one in which one or more hydrogen atoms is replaced by another atom or group of atoms. The Chemistry of Carbon The number of possible carbon-containing compounds is almost limitless. Why are there so many organic compounds? There are several reasons. First, carbon can form stable, covalent bonds with other carbon atoms. Consider three of the allotropic forms of elemental carbon: graphite, diamond, and buckminsterfullerene. Models of these allotropes are shown in Figure.

2 Alkanes: Alkanes are saturated hydrocarbons; that is, alkanes contain only carbon and hydrogen bonded together through carbon-hydrogen and carbon-carbon single bonds. C n H 2n+2 is the general formula for alkanes. In this formula, n is the number of carbon atoms in the molecule. Structure and Physical Properties Three types of formulas, each providing different information, are used in organic chemistry: the molecular formula, the structural formula, and the line formula. The molecular formula tells the kind and number of each type of atom in a molecule but does not show the bonding pattern. Consider the molecular formulas for simple alkanes: The advantage of a structural formula is that it shows the complete structure, but for large molecules it is time-consuming to draw and requires too much space. The compromise is the condensed formula. It shows all the atoms in a molecule and places them in a sequential order that indicates which atoms are bonded to which. The following are the condensed formulas for the preceding five compounds. The simplest representation of a molecule is the line formula. In the line formula we assume that there is a carbon atom at any location where two or more lines intersect. We also assume that there is a carbon at the end of any line and that each carbon in the structure is bonded to the correct number of hydrogen atoms. Compare the structural and line formulas for butane and 2-methylpropane, shown here:

3 Example: Nomenclature The International Union of Pure and Applied Chemistry (I.U.P.A.C.) is the organization responsible for establishing and maintaining a standard, universal system for naming organic compounds. The system of nomenclature developed by this group is called the I.U.P.A.C. Nomenclature System. The following rules are used for naming alkanes by the I.U.P.A.C. system.

4 1. Determine the name of the parent compound, the longest continuous carbon chain in the compound. Notice that these names are made up of a prefix related to the number of carbons in the chain and the suffix -ane, indicating that the molecule is an alkane. Write down the name of the parent compound, leaving space before the name to identify the substituents. Parent chains are high lighted in yellow in the following examples: 2. Number the parent chain to give the lowest number to the carbon bonded to the first group encountered on the parent chain, regardless of the numbers that result for the other substituents. 3. Name and number each atom or group attached to the parent compound. The number tells you the position of the group on the main chain, and the name tells you what type of substituent is present at that position. For example, it may be one of the halogens [F-(fluoro), Cl-(chloro), Br-(bromo), and I-(iodo)] or an alkyl group. In the following examples the parent chain is highlighted in yellow: 4. If the same substituent occurs more than once in the compound, a separate position number is given for each, and the prefixes di-, tri-, tetra-, penta-, and so forth are used, as shown in the following examples: 5. Place the names of the substituents in alphabetical order before the name of the parent compound, which you wrote down in Step 1. Numbers are separated by commas, and numbers are separated from names by hyphens. By convention, halogen substituents are placed before alkyl substituents.

5 Example:

6 ISOMERIC ALKANES: THE BUTANES Methane is the only alkane of molecular formula CH 4, ethane the only one that is C 2 H 6, and propane the only one that is C 3 H 8. Beginning with C 4 H 10, however, constitutional isomers are possible; two alkanes have this particular molecular formula. In one, called n-butane, four carbons are joined in a continuous chain. The n in n-butane stands for normal and means that the carbon chain is unbranched. The second isomer has a branched carbon chain and is called isobutane. HIGHER n-alkanes n-alkanes are alkanes that have an unbranched carbon chain. n-pentane and n- hexaneare n-alkanes possessing five and six carbon atoms, respectively. Their condensed structural formulas can be abbreviated even more by indicating within parentheses the number of methylene groups in the chain. Thus, n-pentane may be written as CH 3 (CH 2 ) 3 CH 3 and n-hexane as CH 3 (CH 2 ) 4 CH 3. This shortcut is especially convenient with longer-chain alkanes. The laboratory synthesis of the ultralong alkane CH 3 (CH 2 )CH 3 was achieved in 1985; imagine trying to write a structural formula for this compound in anything other than an abbreviated way! PROBLEM 2.2 An n-alkane of molecular formula C 28 H 58 has been isolated from a certain fossil plant. Write a condensed structural formula for this alkane. n-alkanes have the general formula CH 3 (CH 2 )xch 3 and are said to belong to a homologous series of compounds. A homologous series is one in which successive members differ by a group. Unbranched alkanes are sometimes referred to as straight-chain alkanes, but, their chains are not straight but instead tend to adopt the zigzag

7 PROBLEM 2.3 Much of the communication between insects involves chemical messengers called pheromones. A species of cockroach secretes a substance from its mandibular glands that alerts other cockroaches to its presence and causes them to congregate. One of the principal components of this aggregation pheromone is the alkane shown in the bond-line formula that follows. Give the molecular formula of this substance, and represent it by a condensed formula. THE C 5 H 12 ISOMERS Three isomeric alkanes have the molecular formula C 5 H 12. The unbranched isomer is, as we have seen, n-pentane. The isomer with a single methyl branch is called isopentane. The third isomer has a three-carbon chain with two methyl branches. It is called neopentane. Table 2.3 presents the number of possible alkane isomers as a function of the number of carbon atoms they contain. As the table shows, the number of isomers increases enormously with the number of carbon atoms and raises two important questions:

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