E. Nomenclature of Alkene Stereoisomers (E/Z and trans/cis) Stereoisomer = compounds with the same molecular formula that differ only in the arrangement of their atoms in space. (Free rotation around double bonds cannot occur unless the double bond is broken and reformed.) (Cyclic alkenes of less than 7 carbons are always cis due to the size of the ring and do not have a cis prefix.) H CH, CH, Critical Thinking Question (CTQ): 18. Write the molecular formula for each of the compounds above. 19. Does the name 2-butene indicate which of the stereoisomers above is being referred to? 20. To indicate the difference between alkenes that have 2 substituents across a double bond, use the prefix trans (across the double bond relative to each other) or cis (same side of the double bond to each other). Thus, the correct IUPAC names for the compounds above are cis-2-butene and trans-2- butene. 21. Add cis or trans where needed to the IUPAC names in the alkene CTQ question above. When more than 2 substituents exist around a double bond, cis and trans can no longer accurately describe the stereoisomers and a different systems is used, the Cahn-Ingold-Prelog convention of E and Z. This system can also be used for alkenes with only 2 substituents. It will be discussed when we cover the chapter on Alkenes and Alkynes later in the semester.
Basics in Organic Reactions Mechanisms
In organic chemistry, the mechanism of an organic reaction is defined as a complete step-by-step explanation of how a reaction of organic compounds happens. A completely detailed mechanism would relate the first structure of the reactants with the last structure of the products and would represent changes in structure and energy all through the reaction step.
Heterolytic Bond Breaking
Heterolytic bond breaking is also known as heterolysis or heterolytic fission or ionic fission. It is defined as breaking of a covalent bond between two different atoms in which one atom gains both of the shared pair of electrons. The atom that gains both electrons is more electronegative than the other atom in covalent bond. The energy needed for heterolytic fission is called as heterolytic bond dissociation energy.
Polar Aprotic Solvent
Solvents that are chemically polar in nature and are not capable of hydrogen bonding (implying that a hydrogen atom directly linked with an electronegative atom is not found) are referred to as polar aprotic solvents. Some commonly used polar aprotic solvents are acetone, DMF, acetonitrile, DMSO, etc.
Oxygen Nucleophiles
Oxygen being an electron rich species with a lone pair electron, can act as a good nucleophile. Typically, oxygen nucleophiles can be found in these compounds- water, hydroxides and alcohols.
Carbon Nucleophiles
We are aware that carbon belongs to group IV and hence does not possess any lone pair of electrons. Implying that neutral carbon is not a nucleophile then how is carbon going to be nucleophilic? The answer to this is that when a carbon atom is attached to a metal (can be seen in the case of organometallic compounds), the metal atom develops a partial positive charge and carbon develops a partial negative charge, hence making carbon nucleophilic.
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