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http://fatmaelif.blogspot.hk/2012/05/how-to-succeed-in-organic-chemistry.html Organic Chemistry 1 Introduction, Alkanes and Structural Isomers IB Topic 10.1.1, 10.1.3 - 10.1.6 p. 366 - 376 1 Starter Activity 1 Give as many meanings as you can for the term “organic”. Name 20 organic substances you have encountered, directly or indirectly, today. 2 Starter Activity 2 THE FOUR CARBON CHALLENGE Build as many molecules as you can that contain 4 carbon atoms. Draw the structure of each one and give its molecular formula. State the name if you think you know it. 3 Starter Activity 2 THE FOUR CARBON CHALLENGE ... continued What “rules” did you follow (if any)? 4 Organic Compounds There are an estimated 100,000,000 or so different “organic compounds”. http://chem1180.blogspot.hk/2010/12/23-introduction-to-organic-chemistry.html All of these compounds contain CARBON ... a very versatile element. 5 What’s So Special About Carbon? Carbon can form either single, double or triple bonds with other carbon atoms or with atoms of other elements. The single carbon-carbon bond is very stable. It is more stable than bonds formed between other Group 14 elements. 6 What’s So Special About Carbon? Carbon can form stable long straight chains, branched chains and ring structures. polyethylene glucose cholesterol 7 Homologous Series A homologous series is a group of organic chemicals with these features: 1. the same general formula 2. successive members differ by CH2 e.g. alkenes = CnH2n first 4 members of the alkenes C2H4 C3H6 C4H8 C5H10 3. chemical properties are the same for all members 4. physical properties gradually change as you move along the homologous series 8 Homologous Series Example: Physical Properties of Alkanes Use your knowledge of structure and bonding to explain the boiling points for these successive alkanes: CH4 = -164ºC C2H6 = -89ºC C3H8 = -42ºC Notice the general formula is the same (CnH2n+2) and all formulas differ by CH2. trend: BP increases moving along the series. explanation: Alkanes are nonpolar molecules. As the total number of electrons increases, the strength of the dispersion forces increases. 9 Homologous Series Common Homologous Series alkanes alcohols amines amides halogenoalkanes aromatics aldehydes ketones alkenes esters alkynes carboxylic acids alcohols nitriles 10 Nomenclature and Structures for Organic Compounds: The Alkanes 11 Hydrocarbons Hydrocarbons are obtained from crude oil. http://heartlandenergydevelopment.blogspot.hk fuels* uses plastics precursor for food additives, dyes, pharmaceuticals, fabrics and more! 12 Homologous Series of Hydrocarbons alkanes single carbon-carbon bonds general formula = CnH2n+2 alkenes at least one double carbon-carbon bond general formula = CnH2n (if one double bond present) alkynes at least one triple carbon-carbon bond general formula = CnH2n-2 (if one triple bond present) 13 IUPAC Nomenclature for Hydrocarbons Names of all hydrocarbons include a stem and a suffix. methane ethene propyne 1. The stem tells you the number of carbons in the longest chain: 1 = meth 2 = eth 3 = prop 4 = but 5 = pent 6 = hex 7 = hept 8 = oct 9 = non 10 = dec 11 = undec 12 = dodec Monkeys eat peeled bananas! (or peanut butter) 2. The suffix tells you the homologous series: ane = alkane ene = alkene yne = alkyne 14 IUPAC Nomenclature for Hydrocarbons Try these: H H H H C C C H propane H H H H H C C C H propyne H H H H C C H ethene 15 IUPAC Nomenclature for Hydrocarbons 3. Many hydrocarbons contain branches of shorter carbon chains. Branch names follow this pattern: stem + yl methyl = 1 carbon branch ethyl = 2 carbon branch The branch name and position is added as a prefix to the name of the longest carbon chain: H H H H 4 3 2 1 H C C C C H H H CH3 H Add the branch name as a prefix to the name of the longest chain. Use a number to indicate the carbon with the branch. Number the carbons in the longest chain starting from the end closest to the branch. 2-methylbutane Always separate numbers from words with hyphens. 16 IUPAC Nomenclature for Hydrocarbons 4. If there are more than two branches of the same length, add a prefix before the branch name and indicate the position of both branches: H H CH3 H 4 3 2 1 H C C C C H 2,2-dimethylbutane H H CH3 H Always number the C atoms from the end closest to a branch. H CH3 H H 4 3 2 1 H C C C C H 2,3-dimethylbutane H H CH3 H 17 IUPAC Nomenclature for Hydrocarbons Try these: H H CH3 H H H C C C C C H 3-methylpentane H H H H H 5-ethyl-2-methylheptane 18 IUPAC Nomenclature for Hydrocarbons 5. The carbon atoms in an alkane may be arranged in a ring structure: Name the alkane by adding “cyclo” as a prefix. cyclobutane methylcyclopentane 19 Molecular, Empirical & Structural Formulas molecular formula shows the actual number of each atom empirical formula shows the lowest ratio of atoms structural formula shows all atoms and bonds C4H10 C2H5 H H H H H C C C C H H H H H 20 Drawing Structural Formulas Complete vs Condensed Structural Formulas Make sure all carbon atoms have 4 bonds! propane pentane CH3CH2CH3 CH3CH2CH2CH2CH3 21 Drawing Structural Formulas Complete vs Condensed Structural Formulas H H CH3 H H 3-methylpentane H C C C C C H H H H H H CH3CH2CH(CH3)CH2 CH3 Branches are shown in parentheses. 22 Drawing Structural Formulas Try these: name condensed structural formula butane CH3CH2CH2CH3 2-methylhexane CH3CH2CH2CH2CH(CH3)CH3 3-ethylpentane CH3CH2CH(C2H5)CH2CH3 2,2-dimethylbutane CH3C(CH3)2CH2CH3 23 Structural Isomers 24 Same or Different? What is similar about these molecules? What is different? same # C and same # H ∴ same molecular formula C5H12 length of longest chain # of branches ∴ different structural formula different names 25 Same or Different? Molecules with the same molecular formula but different structural formulas are called ISOMERS. More specifically, they are called STRUCTURAL isomers when the structural formula is different due to changes in bonding patterns. structural isomers of C5H12 structural isomers of C4H8 structural isomers of C3H6 26 Same or Different? A student draws these structures as examples of structural isomers of C7H16. How many unique structural isomers are shown? (In other words, which structures are the same as another structure?) (Tip: If they have the same name, the structure is the same!) B A C D E F G 27 Same or Different? A student draws these structures as examples of structural isomers of C7H16. How many unique structural isomers are shown? ANSWER = 3 A D = heptane C F B 1 2 3 4 5 6 = 6 3-methylhexane 5 4 3 2 3-methylhexane E 1 = 1 2 3 4 5 6 3-methylhexane G = 3,3-dimethylpentane 28 Properties of Structural Isomers Physical properties of structural isomers are slightly different. C4H10: butane BP = -1.0ºC C4H10: 2-methylpropane BP = -11.7ºC In general, unbranched linear hydrocarbons have higher boiling points (and melting points) than branched hydrocarbons. Linear hydrocarbons are able to “pack together” more tightly than branched hydrocarbons. Because the intermolecular distance is less for the linear molecules, the strength of the intermolecular forces is greater. This increases the BP. 29 Properties of Structural Isomers The greater the number of branches, the lower the BP or MP. C6H14 hexane BP = 68.7ºC 2-methylpentane BP = 60.3ºC 2,2-dimethylbutane BP = 49.7ºC The greater the number of branches, the less closely the molecules can pack together. The strength of the dispersion forces decreases if the molecules are further apart. 30