chem 3

• Alkenes


  Hydrocarbons that contain a carbon–carbon double bond, C = C

  
• Alkenes


  • They occur abundantly in nature
  • e.g. ethylene – plant hormone that induces ripening in fruits

  
• Alkenes

  • α-pinene - a major component of turpentine
  • β-carotene - an orange pigment in some fruits and vegetables, precursor of vitamin A

  
• Alkynes


  Hydrocarbons that contain a carbon–carbon triple bond, C ≡ C

  
• Alkynes


  • The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula CnH2n−2
  • Alkynes are traditionally known as acetylenes, although the name acetylene also refer specifically to C2H2, ethyne

  
• Alkynes


  • Hybridization due to triple bonds allows the uniqueness of alkyne structure
  • Hybridization : sp
  • This triple bond contributes to the nonpolar bonding strength, linear, and the acidity of alkynes

  
• Properties of Alkenes and Alkynes

  • They are nonpolar compounds
  • The only attractive forces between their molecules are very weak London dispersion forces (Van der Waal's forces)
  • Alkenes and alkynes that are liquid at room temperature have densities less than 1.0 g/mL (they float on water)
  • Soluble in nonpolar solvents (kerosene, hexane, CHCl3, CCl4)
  • Insoluble in water
  • Low boiling point and melting point
• Electronic Structure of Alkenes

  • C atoms in a double bond have three equivalent sp2 hybrid orbitals at angles of 120° to one another
  • The fourth C orbital is an unhybridized p orbital perpendicular to the sp2 plane
  • The doubly bonded carbons and the four attached atoms lie in a plane, with bond angles of approximately 120°
  • Sigma (σ) bond is formed by head-on overlap of sp2 orbitals
  • Pi (π) bond is formed by sideways overlap of p orbitals

  
• Electronic Structure of Alkenes

  • Restricted rotation
  • The π bond must break momentarily for rotation around a carbon–carbon double bond to take place, requiring a large amount of energy
  • Energy barrier to rotation around a double bond is estimated 350 kJ/mol (84 kcal/mol), while a single bond is only about 12 kJ/mol

  
• Cis-Trans Isomers of Alkenes

  Cis isomer has the two substituents on the same side of the double bond, trans isomer has the substituents on opposite sides
• Cis-Trans Isomers of Alkenes

  • Cis alkenes are less stable than their trans isomers due to steric (spatial) interference (steric strain) between the large substituents on the same side of the double bond
  • Cis–trans isomerism is not limited to disubstituted alkenes, it occurs whenever each double-bond carbon is attached to two different groups
  • If one of the double-bond carbons is attached to two identical groups, cis–trans isomerism is not possible

  
• Sequence Rules: The E,Z Designation (Cahn – Ingold Prelog)

  1. Assign a priority to the substituents on each carbon of the double bond

  2. E (entgegen, opposite) - higher priority groups are on opposite sides of the double bond

  3. Z (zusammen, together) - higher priority groups are on the same side of the double bond

  
• Sequence Rules: The E,Z Designation (Cahn – Ingold Prelog)


  • Look at the atoms directly attached to each carbon and rank them according to atomic number
  • If a decision can not be reached by ranking the first atoms in the substituents, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found
  • Multiple-bonded atoms are equivalent to the same number of single-bonded atoms

  
• Kinds of Organic Reactions
• Mechanism


  An overall description of how a reaction occurs, describing what takes place at each stage of a chemical transformation, which bonds are broken and formed and in what order, and what are the relative rates of the steps
• Chemical reactions involve bond-breaking in the reactant molecules, bond-making in the product molecules, and the electrons in those bonds must move about and reorganize
• There are two possible ways of bond breaking: symmetrical and unsymmetrical

  
• Mechanism


  • An overall description of how a reaction occurs
  • Describes what takes place at each stage of a chemical transformation
  • Which bonds are broken and in what order
  • Which bonds are formed and in what order
  • What are the relative rates of the steps
• Mechanism: example

  1. Bond-breaking in the reactant molecules

  2. Bond-making in the product molecules

  3. The electrons in those bonds must move about and reorganize
• Bond breaking

  • Symmetrical (homolytic) - one electron remains with each of the product fragment
  • Unsymmetrical (heterolytic) - both electrons remain with one product fragment, leaving the other fragment with a vacant orbital

  
• Bond formation


  • Symmetrical - one electron is donated to the new bond by each reactant
  • Unsymmetrical - both bonding electrons are donated by one reactant
• Radical reaction


  A process that involves symmetrical bond breaking and bond formation
• Polar reaction (Ionic reaction)


  A process that involves unsymmetrical bond breaking and bond formation
• Polar processes are the more common reaction type in organic and biological chemistry
• Effects of bond polarity in chemical reactions

  • Electron-rich sites in one molecule react with electron-poor sites in another molecule or within the same molecule
  • Bonds are made when an electron-rich atom shares a pair of electrons with an electron-poor atom
  • Bonds are broken when one atom leaves with both electrons from the former bond
• Nucleophile


  • A substance that is "nucleus loving" and thus attracted to a positive charge
  • Has a negatively polarized, electron-rich atom
  • Can form a bond by donating an electron pair to a positively polarized, electron-poor atom
  • Can be either neutral or negatively charged and usually have lone-pairs of electrons
• Nucleophiles


  • Strong nucleophiles (usually anions with a full negative charge)
  • Neutral nucleophiles
• Electrophile


  • Is "electron-loving"
  • Has a positively polarized, electron-poor atom and can form a bond by accepting a pair of electrons from a nucleophile
  • Can be either neutral or positively charged
• Addition of HCl to Ethylene

  1. Ethylene: H2C = CH2

  2. Carbon-carbon double bond results from orbital overlap of two sp2-hybridized carbon atoms

  3. The π part of the double bond results from sp2–sp2 overlap

  4. The σ part of the double bond results from p–p overlap

  5. The valence electrons in alkanes are relatively inaccessible because they are tied up in strong, nonpolar C - C and C - H bonds between nuclei

  6. The π electrons in alkenes are accessible to external reagents because they are located above and below the plane of the double bond rather than between the nuclei

  7. An alkene π bond is much weaker than an alkane σ bond

  8. Alkenes typically react by donating an electron pair from the double bond to form a new bond with an electron-poor, electrophilic partner

  9. Hydrogen chloride: H - Cl

  10. As a strong acid, HCl is a powerful proton (H+) donor and a good electrophile

  11. The reaction of HCl with ethylene is therefore a typical electrophile–nucleophile combination as in all polar reactions

  
• Transition States and Intermediates


  1. Reactant molecules must collide

  2. Reorganization of atoms and bonds occur

  3. As H2C = CH2 and HCl molecules crowd together, their electron clouds repel each other, causing the energy level to rise

  4. If the collision has occurred with sufficient force and proper orientation, the reactants continue to approach each other despite the repulsion, until the new C – H bond starts to form and the H – Cl bond starts to break

  5. At some point, a structure of maximum energy is reached, called the transition state

  6. The transition state represents the highest-energy structure involved in the step of the reaction, and cannot be isolated or directly observed

  7. The transition state is a kind of activated complex of the two reactants, where the C = C bond is partially broken and the new C – H bond is partially formed

  8. Eact is the energy difference between the reactants and the transition state, and is a measure of how rapidly the reaction occurs

  9. Large Eact results in a slow reaction because few of the reacting molecules collide with enough energy to reach the transition state

  10. Small Eact results in a rapid reaction because almost all reacting molecules are energetic enough to climb to the transition state

  
• Eact


  The energy difference between the reactants and the transition state, a measure of how rapidly the reaction occurs
• Large Eact


  Results in a slow reaction because few of the reacting molecules collide with enough energy to reach the transition state
• Small Eact


  Results in a rapid reaction as almost all reacting molecules are energetic enough to climb to the transition state
• Eact of most organic reactions is in the range of 40 to 125 kJ/mol or 10 to 30 kcal/mol
• Reactions with activation energies less than 80 kJ/mol take place at or below room temperature
• Reactions with higher activation energies often require heating to give the molecules enough energy to climb the activation barrier
• Transition state

  1. Energy is released as the new C – H bond forms fully

  2. Curve turns downward until it reaches a minimum
• Reaction intermediate


  The carbocation formed transiently during the course of the multistep reaction
• Second step of reaction

  1. Eact2 and transition state

  2. Activated complex between the electrophilic carbocation intermediate and nucleophilic Cl– anion

  3. New C – Cl bond is partially formed

  4. Curve turns downward as the C – Cl bond forms fully to give the final addition product
• Each individual step in the reaction has its own energy change