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Chapter 15

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University of Toronto Mississauga
Judith C Poe

Chapter 15: Organic chemistry 4/8/2013 6:55:00 AM  Most organic molecules have more complex structures than most inorganic molecules  Electron configuration, electronegativity and covalent bonding:  Carbon’s formation of covalent bonds rather than ionic bonds is the result of its electron configuration and its electronegativity value  Carbon’s ground state configuration is [He] 2s 2p2 2 4+  The loss of four electrons to form the C  cation requires energy equal to the sum If the IE t1rough IE , t4e 4- gain of four electrons to form the C anion requires the sum of EA 1 through EA 4 the last three steps of which are endothermic  Lying at the center of period 2, carbon has an electronegativity, EN=2.5, that is midway between that of the most metallic element and the most non-metallic element  Bond properties, catenation and molecular shape:  The number and strength of carbon’s bond lead to its property of catenation  Catenation is the ability to bond to itself  Carbon’s small size and ability to form hybrid orbitals and multiple bonds increase the number of different molecules by affecting molecular shape  The small size of carbon allows close approach to another atom and thus greater orbital overlap, so forming short strong bonds  The C-C bond is short enough to allow side to side overlap of half- filled unhybridized p orbitals and the formation of multiple bonds, which restrict rotation of attached groups  Molecular stability  Although silicon and several other elements also catenate, none can form chains as stable as those of carbon  Atomic size and bond strength: as atomic size increases down the group 4, bonds between identical atoms become longer and weaker  Relative enthalpies of reaction: a C-C and a C-Cl have nearly the same energy so relatively little heat is released when a C chain reacts and one bond replaces the other  Orbitals available for reaction: unlike C, Si has low-energy d orbitals that can be attacked by lone pairs of incoming reactants  The chemical diversity of organic compounds is founded on atomic and bonding behavior and is due to three related factors (Bonding to heteroatoms, electron density and reactivity and importance of functional groups  Heteroatoms are atoms other than C and H  Many compounds contain heteroatoms, most common of which are N and O  Most reactions starts when a region of high electron density on one molecule meets a region of low electron density on the other  These regions may be due to the presence of multiple bond properties of carbon-heteroatom bonds 15.2  The carbon-carbon bond form the skeleton  Carbon skeleton the longest continual chain is the backbone and any branches are the limbs  Hydrocarbons are the simplest type of organic compound containing only C and H  Groups joined by a single (sigma) bond are free to rotate  As the total number of C atoms increases the number of different arrangements increases as well  The arrangement of C atoms determines the skeleton so a straight chain and bent chain represent the same skeleton  Hydrocarbons can be classified into four main groups  Alkanes:  A hydrocarbon that only contains a single bond  CnH 2n+2  The alkanes comprise a homologous series, differing be a CH 2  In an alkane, each C is hybridized  Alkanes are saturated hydrocarbons  The expanded formula shows each atom and bond  A cyclic hydrocarbon contains one or more rings in structure  When a straight chain alkane forms a ring, two H atoms are lost CnH 2n  Cycloalkanes are non-planar because of the need to minimize electron repulsion between adjacent H atoms  As a result, orbital overlap of adjacent C atoms is maximized  Compounds with the sane molecular formula but different properties are called isomers  Those with different arrangements of bonded atoms are structural (or constitutional) isomers  Alkanes are nearly non-polar, their physical properties are determined by dispersion forces  The more spherical members boils lower than the more elongated one, because a spherical shape leads to less intermolecular contact and thus lower total dispersion forces that does an elongated shape  The greater the intermolecular contact the stronger the dispersion forces and the higher the boiling point  Stereoisomers are molecules with the same arrangement of atoms but different orientation of groups in space.  Optical isomerism is one type of stereoisomerism  Optical isomerism; when two objects are mirror images of each other and cannot be superimposed  Optical isomers are also called enantiomers  An asymmetric molecule is called chiral  An organic molecule is chiral if it contains a carbon atom that is bonded to four different groups  Properties of optical isomers: optical isomers are identical in all but two aspects o In their physical properties: optical isomers differ only in the direction that each isomer rotates the plane of polarized light o A polarimeter is used to measure the angle that the plane rotated o An optical isomer is optically active because it rotates the plane of this polarized light o The dextrorotatory isomer rotates the plane of light clockwise o The levorotatory isomer rotates the plane anticlockwise o An equimolecular mixture of two isomers (called a racemic mixture) does not rotate the plane of light because the dextrorotation cancels out the levorotation o The specific rotation is a characteristic measurable property of an optical isomer at a certain temperature, concentration and wavelength of light o In their chemical properties, optical isomers differ only in a chiral (asymmetric) chemical environment. One isomer of optically active reactant is added to a mixture of optical isomers of another compound. The products of the reaction have different properties and can be separated  The role of optical isomerism in organisms and medicines: o an organism can utilize only one of a pair of optical isomers because of its enzymes. An enzyme distinguishes one optical isomer from another because its binding site is chiral (asymmetric). The shape of optical isomer fits at the binding site but the mirror image shape of the other isomer does not fit so it cannot bind  Alkenes: A hydrocarbon that contain at least one C=C bond is called an  alkene, C n 2n 2  The double bonded C atoms are sp hybridized  Unsaturated hydrocarbons  The C=C bond and geometric (cis-trans) isomerism: o There are two major structural differences between alkanes and alkenes i. Alkanes have a tetrahedral geometry around each C atom, whereas alkenes are trigonal planar ii. The C-C bond allows rotation of bonded groups, so that the atoms in an alkane continually change their relative positions, in contrast the pi bond of the alkene restricts rotation so the relative position of the atoms attached to the double bond are fixed o Restriction leads to another type of stereoisomerism; geometric isomerism o Also known as the cis-trans isomerism o Geometric isomerism have different orientation of groups around a double bond o CIS Same side of the double bond o TRANS opposite sides of the double bond o Causes difference in molecular shape and physical properties  Alkynes  Hydrocarbons that contain at least one Carbon to carbon triple bond  C n 2n-2  Each C is sp hybridized  Alkynes are much more reactive than alkanes  Aromatic hydrocarbons:  Unlike the cycloalkanes, aromatic hydrocarbons are planar molecules  Usually with one or more rings of six C atoms and are often with alternating single and double bonds  Benzene is also shown as a resonance hybrid, with a circle or a dashed circle representing the delocalized character of the pi electrons  The position of the two groups are indicated by o-(ortho) for groups on adjacent ring C atoms, m- (meta) for groups separated by one ring C atoms and p- (para) for groups on opposite ring C atoms  Boron silicon and sulfur also form catenated hydrides but these are unstable 15.3  We use R- to signify a general organic group attached to one of the atoms shown  You can view R- as an alkyl group  Three important reaction types are addition, elimination and substitution  Can be identified by comparing the number of bonds to C in reactants and products  Addition reactions  Occurs when an unsaturated reactant becomes a saturated product  The C atoms are bonded to more atoms in the product than in the reactant  The C=C, C=C, and C=O bond commonly undergo addition  The pi bond break, leaving the sigma bond intact. In the product, the two C atoms or C and O form two additional sigma bonds  Addition reaction is exothermic  Elimination reaction:  Elimination reaction are the opposite of addition reactions  The occur when a saturated reactant becomes an unsaturated product  C atoms are bonded to fewer atoms in the products than in the reactants  An H atom and a halogen group, or an H atom and an –OH group are typically eliminated, C atoms are not  The driving force of many elimination reactions is the formation of a small stable molecule (which increases the entropy of the system  Substitution reaction:  A substitution reaction occurs when an atom (or group) from an added reagent substitutes for one attached to a carbon in the organic reactant  The C atom is bonded to the sane number of atoms in the product as in the reactant  Can be saturated or unsaturated  An important process in many organic reactions is oxidation-reduction  A more electronegative atom takes some electron density from the C, whereas a less electronegative atom gives some electron density to the C.  When a C atom in the organic reactant forms more bonds to O or fewer bonds to H, thus losing some electron density the reactant is oxidized and the reaction is called oxidation  When a C atom in the organic reactant forms fewer bonds to O, or more bonds to H thus gaining more electron density, the reactant is reduced and the reaction is called reduction 15.4  The central organizing principle of organic chemistry is the functional group  The distribution of electron density in a functional group is a major factor in the reactivity of the compound  The electron density can be high as in C=C and C=C bonds, or it can be low at one end of a bond and high at the other end as in C-Cl and C-O  Electron rich or polar ponds enhance the oppositely charged pole in the reactant. So the reactant attract each other and begin a sequence of bond forming and bond breaking steps that lea
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