Orgo Notes - Chapter 4 - Summary Organic Chemistry PDF

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Chapter 4 Notes...

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4.1 Structure and Bonding in Alkenes A. Carbon Hybridization in Alkenes Sp2 hybridized – 33% s character + 67% p character Electron density is concentrated a little closer in an sp2 orbital than in an sp3 orbital A 2pz orbital is left over (could be 2px or 2py, doesn’t matter which) B. The Π (pi) Bond The left over 2p orbitals form the second bond of the double bond, the pi bond The 2p orbit is one orbital with 2 lobes Pi electrons generally have higher energy than sigma electrons Pi electrons are more easily removed than sigma electrons Pi bonds are weaker than sigma bonds This is because pi bonds overlap side-to-side Bonds with more s character are shorter C. Double-Bond Stereoisomers Constitutional isomers – molecules that have the same molecular formula but differ in connectivity Stereoisomers – compounds with identical connectivities that differ in the spatial arrangement of atoms Double-Bond Stereoisomers = Cis-trans Stereoisomers = E,Z-Stereoisomers – compounds related by an internal rotation of 180⁰ about the double bond Cis – groups on the same side Also Z Trans – groups on opposite sides Also E To change from cis to trans, a pi bond must be broken and then the molecule must be rotated around the sigma bond This requires energy and does not happen spontaneously Stereocenter – both carbons of the double bond in stereoisomers 4.2 Nomenclature of Alkenes A. IUPAC Substitutive Nomenclature The principle chain is the carbon chain containing the greatest number of double bonds -ene = 1 double bond -adiene = 2 double bonds -atriene = 3 double bonds Vinyl H2C=CH– Allyl H2C=CH–CH2– Isopropenyl H2C=C– | CH3 Substituent groups are numbered from the point of attachment to the principal chain To name substituents, drop the e from the end of the alkene name and add a –yl Some alkenes have nonstandard names Styrene Ph–CH=CH2 Isoprene H2C=C–CH=CH2 | CH3 B. Nomenclature of Double-Bond Stereoisomers: The E,Z System 1. Examine the atoms directly attached to a given carbon of the double bond, and then either Assign higher priority to the group containing the atom of higher atomic number Or assign higher priority to the group containing the isotope of higher atomic mass

2. If the atoms directly attached to the double bond are the same, then, working outward from the double bond, consider within each group the set of attached atoms. You’ll have two sets, one for each group on the double bond. Arrange the attached atoms within each set in descending priority order, and make a pairwise comparison of the atoms in the two sets. The higher priority is assigned to the atom of higher atomic number (or atomic mass in the case of isotopes) at the first point of difference. 3. If the sets of attached atoms are identical, move away from the double bond within each group to the next atom following the path of highest priority, and identify new sets of attached atoms. Then apply rule 2 to these new sets. Keep following this step until a decision is reached. Remember that a priority decision must be made at the first point of difference. For double bonds, you must duplicate what’s at each end of the double bond Triple what’s at each end of a triple bond 4.3 Unsaturation Number The molecular formula of an organic compound contains “built-in” information about the number of rings and double (or triple) bonds Unsaturation number = Degree of unsaturation = U = a quantity that indicates the presence of rings or double bonds within a molecule This number is the average of the possible number of hydrogens and the actual number of hydrogens (or other halogens)

U=

2 C+2−H =number of rings ∧double bonds 2

When nitrogen is in an organic compound, the formula changes a little bit Each nitrogen can have a total of 3 hydrogens rather than 4, so the saturated number increases by only 1 for each nitrogen rather than the 2 for each carbon

U=

2 C+2+ N −H 2

4.4 Physical Properties of Alkenes Alkenes differ little in their physical properties from the corresponding alkanes other than in melting point and dipole moments Alkenes are flammable, nonpolar, less dense than water, and insoluble in water Alkenes of lower molecular weight are gases at room temperature An sp2 carbon is somewhat more electronegative than an sp3 carbon The electron density in an sp2 orbital lies closer to the nucleus than it does in an sp3 orbital This means that any sp2–sp3 carbon-carbon bond has a small bond dipole in which the sp3 carbon is the positive end and the sp2 carbon is the negative end of the dipole 4.5 Relative Stabilities of Alkene Isomers A. Heats of Formation ΔH (reaction) = H (products) – H (reactants) B. Relative Stabilities of Alkene Isomers An alkene is stabilized by alkyl substituents on the double bond The alkene with the greatest number of alkyl substituents on the double bond is usually the most stable Sp2-sp3 carbon-carbon bond is stronger than an sp3-sp3 carbon-carbon bond Increasing bond strength lowers the heat of formation The s orbital is at a lower energy than a p orbital Increasing s character in a bond involves electrons at lower energy levels  stronger bond Bond strength increases with the fraction of s character in the component hybrid orbitals 4.6 Addition Reactions of Alkenes

4.7 Addition of Hydrogen Halides to Alkenes Addition of H-F, H-Cl, H-Br, or H-I  Alkyl halide = compound where a halogen is bound to a saturated carbon atom

Bond forms on the less substituted carbon first A. Regioselectivity of Hydrogen Halide Addition The main product is that isomer in which the halogen is bonded to the carbon of the double bond with the greater number of alkyl substituents, and the hydrogen is bonded to the carbon with the smaller number of alkyl substituents The number of substituents is more important than the size of the substituents Regioselective Reaction – a reaction where the products could consist of more than one constitutional isomer and one of the possible isomers is found in excess over the other When the 2 carbons of a double bond have equal numbers of alkyl substituents, little or no regioselectivity is observed in hydrogen halide addition, even if the alkyl groups are of different size Markovnikov’s Rule: The halogen of a hydrogen halide attaches itself to the carbon of the alkene bearing the lesser number of hydrogens and greater number of carbons B. Carbocation Intermediates in Hydrogen Halide Addition

A carbon of the pi bond is protonated This is an electron-pair displacement reaction

A halide ion reacts with the resulting carbocation This is a Lewis acid-base association reaction These reactions produce reactive or unstable intermediates = species that react so rapidly that they never accumulate in more than very low concentration

The formation of tert-butyl cation is much faster than the formation of isobutyl cation, so the tert-butyl cation is effectively the only one formed C. Structure and Stability of Carbocations Primary carbocation – has one alkyl group bound to the electron-deficient carbon Secondary carbocation – has two alkyl group bound to the electron-deficient carbon Tertiary carbocation – has three alkyl group bound to the electron-deficient carbon Alkyl substituents at the electron-deficient carbon strongly stabilize carbocations Stability: tertiary > Secondary > Primary Hyperconjugation – overlap of bonding electrons from the adjacent sigma bonds with the unoccupied 2p orbital of a carbocation Stabilizes carbocations by alkyl branching Summary: Alkene double bond is protonated at the carbon with the fewer alkyl substituents so that a more stable carbocation is formed The one with the greater number of alkyl substiuents at the electron-deficient carbon Halide ion reacts with the electron-deficient carbon D. Carbocation Rearrangement in Hydrogen Halide Addition Rearrangement – when a group from the starting material moves to a different prosition in the product

Rearrangement occurs to increase stability of the carbocation intermediate Rearrangements involving the movement of hydrogen (with its 2 electrons) is called Hybride Shif Rearrangements only occur between adjacent carbons Summary A rearrangement almost always occurs when a more stable carbocation can result A rearrangement that would give a less stable carbocation generally doesn’t occur The group that migrates in a carbocation rearrangement comes from a carboe directly attached to the electron-deficient, positively charged carbon of the carbocation The group that migrates in a rearrangement is typically an alkyl group, aryl group, or a hydrogen When there is a choice between the migration of an alkyl group (or aryl group) or a hydrogen from a particular carbon, hybride migration typically occurs because it gives the more stable carbocation 4.8 Reaction Rates A. The Transition State Rate – the number of reactant moleucles converted into product in a given time Transition state – unstable state of maximum free energy Has a higher energy than either the reactants or products  This is the energy barrier - Standard free energy of activation = difference between standard free energy of the transition state and the reactants Higher the barrier, the smaller the rate A reaction and its reverse have the same transition state B. The Energy Barrier

For every increment of 2.3RT (5.7 kJ/mol at 298 K) difference in standard free energies of activation, the rates of 2 reactions differ by a factor of 10 Reaction rates are very sensitive to their standard free energies of activation Rate of a reaction is directly related to the fraction of molecules that has enough energy to cross the energy barrier - means that raising the temperature will speed up reactions Equilibrium constant of a reaction tells us absolutely nothing about its rate

Summary The size of the energy barrier, or standard free energy of activation Reactions with smaller are faster The temperature Reactions are faster at higher temperatures C. Multistep Reactions and the Rate-Limiting Step Each free-energy maximum between reactants and products in a reaction free-energy diagram represents a transition state, and the minimum represents the carbocation intermediate Rate-Limiting Step – the slowest step in a multistep chemical reaction The rate of the overall reaction is equal to the rate of the rate-limiting step The rate-limiting step is the step with the transition state of highest free energy (highest hump) Anything that increases the rate of the rate-limiting step increases the overall reaction rate If a change in the reaction conditions affects the rate of the reaction, it is the effect on the ratelimiting step being observed D. Hammond’s Postulate For a reaction in which an intermediate of relatively high energy is either formed from reactants of much lower energy or converted into products of much lower energy, the structure and energy of the transition state can be approximated by the structure and energy of the intermediate itself Simplified: the structures and energies of the transition states are approximated by the structures and energies of the unstable intermediates, the carbocations 4.9 Catalysis Catalyst – substance that increases the rate of a reaction without being consumed A catalyst increases the reaction rate, which means that it lowers the standard free energy of activation for a reaction A catalyst is not consumed It may be consumed in one step of a catalyzed reaction, but if so, it is regenerated in a subsequent step A catalyst that strongly accelerates a reaction can be used in very small amounts A catalyst doesn’t affect the energies of reactants and products A catalyst doesn’t affect the ΔG of a reaction and, consequently, doesn’t affect the equilibrium constant A catalyst accelerates both the forward and reverse of a reaction by the same factor Heterogeneous catalyst – when a catalyst and reactants exist in separate phases Homogoeneous catalyst – a catalyst that is soluble in a reaction solution A. Catalytic Hydrogenation of Alkenes Catalytic hydrogenation – an addition of hydrogen to an alkene in the presence of a catalyst One of the best way sto convert alkenes into alkanes

B. Hydration of Alkenes Acid catalyzed hydration  alcohol

Because water is present, the actual acid is the hydrated proton (H 30+)

Whenever H30+ acts as an acid, its conjugate base H20 acts as the base Acids and their conjugate bases always act in tandem in acid-base catalysis

C. Enzyme Catalysis Enzyme = biological catalysts...