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ORGANIC CHEMISTRY CHM 207

ORGANIC CHEMISTRY CHM 207. CHAPTER 1: INTRODUCTION TO ORGANIC CHEMISTRY. NOR AKMALAZURA JANI. SUBTOPICS. Characteristic features of organic compounds. Electronegativity and chemical bonds. Isomerism. Reaction of organic compounds. ORGANIC CHEMISTRY. Organic chemistry:

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ORGANIC CHEMISTRY CHM 207

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  1. ORGANIC CHEMISTRY CHM 207 CHAPTER 1: INTRODUCTION TO ORGANIC CHEMISTRY NOR AKMALAZURA JANI

  2. SUBTOPICS • Characteristic features of organic compounds. • Electronegativity and chemical bonds. • Isomerism. • Reaction of organic compounds.

  3. ORGANIC CHEMISTRY • Organic chemistry: • The branch of chemistry that deals with carbons compounds. • Also contain element such as O, N, P, S and halogen (F, Cl, Br, I) • ‘Organic’ – derived from living organisms • Study of compounds extracted from living organisms and their natural products. • Examples: sugar, starch, urea, waxes, carbohydrates, fats and etc • Human are composed of organic molecules – proteins in skin, lipid in cell membranes, glycogen in livers and the DNA in the nuclei of cells.

  4. Chemistry of carbon: - Two stable isotops (13C and 12C) - electron configuration: 1s2 2s2 2p2 - four valence electrons - can form more compounds than any other element - able to form single, double and triple carbon-carbon bonds, and to link up with each other in chains and ring structures

  5. Characteristic features of organic compounds • Bonding hybridization theory i) sp hybrid orbitals - combine a s orbital and a p orbital on the same atom - hybrid orbitals give a bond angle of 180o - linear bonding arrangement - example: acetylene (contains carbon triple bond) -

  6. Bonding in Acetylene, C2H2 180o

  7. Cl 0 lone pairs on central atom Be Cl 2 atoms bonded to central atom

  8. sp2 hybrid orbitals - when an s orbital combines with two p orbitals, it will formed three hybrid orbitals and oriented at 120o angles to each other. - called sp2 hybrid orbitals (composed of one s orbital and two p orbitals). - the 120o arrangement is called trigonal geometry. 121.7o 116.6o Ethylene, close to 120o

  9. sp3 hybrid orbitals - when an s orbital combines with three p orbitals, it will formed four hybrid orbitals and oriented at 109.5o angles to each other. - called sp2 hybrid orbitals (composed of one s orbital and three p orbitals). - the arrangement is called tetrahedral.

  10. Hydridization of other atoms: Nitrogen and oxygen - type of hybridization: sp3 - bond angle for N-H : 107.3o - bond angle for O-H : 104.5o

  11. # of atoms bonded tocentral atom # lone pairs on central atom Arrangement ofelectron pairs Molecular Geometry Class linear linear B B Valence shell electron pair repulsion (VSEPR) model: Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs. AB2 2 0 A = central atom B = surrounding atom E = lone pair on A

  12. trigonal planar # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar Arrangement ofelectron pairs Molecular Geometry Class VSEPR AB2 2 0 linear linear AB3 3 0

  13. Boron trifluoride (BF3)

  14. # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar trigonal planar AB3 3 0 Arrangement ofelectron pairs Molecular Geometry Class tetrahedral tetrahedral VSEPR AB2 2 0 linear linear AB4 4 0

  15. # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar trigonal planar AB3 3 0 Arrangement ofelectron pairs Molecular Geometry Class trigonal bipyramidal trigonal bipyramidal VSEPR AB2 2 0 linear linear tetrahedral tetrahedral AB4 4 0 AB5 5 0

  16. # of atoms bonded tocentral atom # lone pairs on central atom trigonal planar trigonal planar AB3 3 0 Arrangement ofelectron pairs Molecular Geometry Class trigonal bipyramidal trigonal bipyramidal AB5 5 0 octahedral octahedral VSEPR AB2 2 0 linear linear tetrahedral tetrahedral AB4 4 0 AB6 6 0

  17. # of atoms bonded tocentral atom # lone pairs on central atom Arrangement ofelectron pairs Molecular Geometry bent Class trigonal planar VSEPR trigonal planar trigonal planar AB3 3 0 AB2E 2 1

  18. # of atoms bonded tocentral atom # lone pairs on central atom trigonal pyramidal Arrangement ofelectron pairs Molecular Geometry AB3E 3 1 tetrahedral Class bent tetrahedral VSEPR tetrahedral tetrahedral AB4 4 0 AB2E2 2 2

  19. # of atoms bonded tocentral atom # lone pairs on central atom trigonal bipyramidal distorted tetrahedron Arrangement ofelectron pairs Molecular Geometry AB4E 4 1 Class trigonal bipyramidal T-shaped AB3E2 3 2 linear trigonal bipyramidal VSEPR trigonal bipyramidal trigonal bipyramidal AB5 5 0 AB2E3 2 3

  20. octahedral octahedral AB6 6 0 # of atoms bonded tocentral atom # lone pairs on central atom square pyramidal octahedral AB5E 5 1 Arrangement ofelectron pairs Molecular Geometry Class square planar octahedral VSEPR AB4E2 4 2

  21. Class Molecular Geometry AB2 linear AB3 trigonal planar AB2E bent AB4 tetrahedral AB3E trigonal pyramidal AB2E2 bent AB5 trigonal bipyramidal AB4E distorted tetrahedron AB3E2 T-shaped AB2E3 linear AB6 octahedral AB5E square pyramidal AB4E2 square planar

  22. ELECTRONEGATIVITY AND CHEMICAL BONDS • Electronegativity: a measure of the force of an atom’s attraction of electrons that is shares in a chemical bond with another atom. • Electronegativity is used to estimate the degree of ionic or covalent character in a chemical bond. • A non polar covalent bond is one in which the difference in electronegativity between the bonded atoms is 0.4 or less. - example: bond between C and H is classified as nonpolar covalent because the difference in electronegativity between two atoms is 0.4 unit on the Pauling scale.

  23. A polar covalent bond is one in which the difference in electronegativity between two atoms is between 0.5 and 1.8. • - example: bond between H-Cl (electronegativity between two atoms is 0.9 unit)

  24. F H F H d- d+ e- poor e- rich electron rich region electron poor region d+ d- cross arrow = indicate the direction of bond polarity

  25. The Electronegativities of Common Elements

  26. ISOMERISM • Isomers: organic compounds that have same molecular formula but different arrangements of atoms. • Isomerism: the existence of two or more organic compounds with the same molecular formula but different arrangements of atoms. • Two types of isomerism: i) constitutional isomerism ii)stereoisomerism

  27. CONSTITUTIONAL (STRUCTURAL) ISOMERISM • Molecules that have same molecular formula but have different structural formulae. • Subdivided into 3 different categories: i) Chain isomerism ii)Positional isomerism iii)Functional group isomerism

  28. Chain isomerism • Chain isomers are those which differ in the structure of their carbon chains, differ in the length of their straight chains or branch. • Example: - alkane with molecular formula C4H10 has 2 chain isomers

  29. - alcohols with the formula C4H9OH • Possess the same functional group, belong to the same homologous series. • Different physical properties. • Similar chemical properties.

  30. Positional isomerism • Have the same carbon skeleton and belong to the same homologous series, but differ position of the functional group. • Similar chemical properties because have the same functional group. • Different physical properties. • Examples: i) bromoalkanes with the molecular formula C3H7Br

  31. ii) Alcohols with the molecular formula C3H7OH iii) Alkenes with the molecular formula C5H10

  32. Aromatic compounds. - If two bromine atoms replace two hydrogen atoms to form disubstituted benzene, three isomers can be formed.

  33. Isomers which have the same molecular formula but contain different functional groups. Same molecular formula but belong to different homologous series. Different chemical and physical properties. Functional Group Isomerism

  34. Examples

  35. STEREOISOMERISM • Isomers with same structural formula but they have different spatial arrangements of their atoms. • Divided into two categories: i) cis-trans isomerism ii)optical isomerism

  36. cis-trans (geometrical) isomerism • Occurs in compounds in which free rotation is prevented by the presence of a carbon-carbon double bond or a cyclic structure which hinders or obstructs the rotation of a C-C single bond in the ring. • Known as geometrical isomerism. • Same structural formula, differ only in how the atoms or groups are orientated in space. • Example: - alkenes: 2-butene exists has two cis-trans isomers.

  37. cis-trans (geometrical) isomerism • Have same functional group, thus have same chemical properties. • Have different physical properties as a result of the different spatial arrangements of the groups. • For example: - boiling point of cis-2-butene is 3.7oC but boiling point of trans-2-butene is 0.9oC. • cis isomers have higher bp compared to trans isomer due to more polarised nature. • trans isomer have higher melting points - more linear structure that enables them to form a more densely-packed crystalline structure. μ = 0.33 D μ = 0 D

  38. cis-trans (geometrical) isomerism • Cannot occur if one of the carbons atoms in the double bond or the ring structure has two identical atoms or groups. • Example: - 1,2-dichloroethene shows cis-trans isomerism but 1,1-dichloroethene and 1,1,2-trichloroethene do not show cis-trans isomerism.

  39. Examples of cyclic compounds that show cis-trans isomerism:

  40. Optical isomerism • Optical activity is the ability of certain crystals or solutions of certain substances to rotate the plane of plane-polarised light. • The substances are said to be optically active. • Optical isomers: optically active substances which possess the same structural formula but differ in their effect on plane-polarised light. • Isomers which rotates the plane of polarisation to the right (in clockwise direction) – dextrorotatory isomer or the (+)isomer. • Isomer that rotates the plane of polarisation to left (anti-clockwise direction) – laevorotatory or the (-)isomer. Dextrorotatory (+) laevorotatory (-)

  41. CHIRALITY • Chiral: - molecule that have different mirror image (nonsuperimpose). - must have an enantiomers. • Achiral: - a molecule that do not have chiral carbon - identical with its mirror image - optically inactive - not chiral

  42. CHIRALITY CENTRE AND ENANTIOMERS • An organic molecule will exhibit optical isomerism or optical activity if it contains at least one chiral carbon atom. • Chiral carbon atom or asymmetric carbon atom (C*): carbon atom attached to four different atoms or groups. • Enantiomers / optical isomers: - A molecule that contains an asymmetric carbon atom and have a mirror image that cannot be superimposed on it. - have same structural formula but different spatial arrangement of the atoms. - optically active. • Diastreoisomers: - stereoisomers that are not mirror image of each other. - include cis-trans isomers.

  43. Write the expanded formula for each of these molecules and mark the chiral carbon or stereogenic centre in each of the molecule with an asterisk (*): • 2-bromopentane • 1,2-dibromobutane • 4-ethyl-4-methyloctane

  44. THREE-DIMENSION (3D) FORMULA

  45. RACEMATE • Racemate or racemic mixture: - an equimolar mixture (+) and (-) isomers. • Does not rotate the plane of polarised light (optically inactive). • Example: 50% (+)-gliseraldehyde + 50% (-)-gliseraldehyde = (±)-gliseraldehyde

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