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Why are saturated fatty acids solid at room temperture?
They pack together well
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Why are cis- unsaturated fatty acid chains liquid at room temperature?
They do not pack togther well
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What is the common name for alkynes?
acetylenes
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What is the old name for Alkenes?
Olefins
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How do you determine higher priority in the Cahn-Ingold-Prelog convvention?
The higher the atomic number, the higher priority
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What is the (E) configuration?
the highest priority groups are on the opposite side of the double bond
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What does the 'E' stand for in the Cahn-Ingold-Prelog Convention?
entgegen
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What is the (Z) configuration?
The highest priority groups are on the same side of the double bond
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What does the 'Z' stand for in the Cahn-Ingold-Prelog Convention?
zusammen
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Which configuration of alkenes is more stable?
Trans
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What increases the stability of alkenes?
The greater the number of attached alkyl groups, the greater the alkenes stability
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Cycloalkenes containing 5 carbon atoms or fewer exist only in _____ form
cis-
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Why have Trans-cyclohexene and trans-cycloheptene not been iscolated?
Because they have a very short lifetime
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What mechanism does dehydrohalogenation promote?
E2 mechanism
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How to favor an E2 mechanism?
- Use a secondary or tertiary alkyl halide if possible (because steric hinderance in the substrate will inhibit substitution)
- When a snthesis must begin with a primary alkyl halide, use a bulky base (because the steric bulk of the base will inhibit substitution)
- Use a high concentration of a strong and nonpolarizable base, such as an alkoixide (because a weak and polarizable base would not drive the reaction toward a bimolecular reaction, thereby allowing unimolecular processes (such as SN1 or E1 reactions) to compete
- Sodium ethoxide in ethabol and potassium tert-butoxide in tertbutyl alcohol are bases typically used to promote E2 reactions
- Use elevated temperature because heat generally favors elimination over substitution (because elimination reactions are entropically favored over substitution reactions)
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Why do you use a secondary our tertiary alkyl halide when favoring an E2 mechanism?
Because steric hinderance in the substrate will inhibit substitution
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Why should you use a bulky base when a synthesis must begin with a primary alkyl halide when favoring E2 mechanisms?
Because the steric bulk of the base will inhibit substitution
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Why must you use a high concentration of a strong and non polarizable base, such as an alkoxide when favoring E2 mechanisms?
Because a weak and polarizable base would not drive the reaction toward a bimolecular reaction, thereby allowing unimolecular processes (such as SN1 or E2 reactions) to compete
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Which solutions are typically used for favoring E2 mechanisms?
- Sodium ethoxide in ethanol (EtONa/EtOH)
- Potassium tert-butoxide in tertbutyl alcohol (t-BuOK/t-BuOH)
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Why must you use an elevated temperature when favoring E2 mechanisms?
Elimination reactions are entropically favored over substitution reactions
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What is Zaitsev's rule?
If more than one alkene can be formed during dehalogenation by an elimination reaction, the more stable alkene is the major product
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What happens when a small base is used for dehydrohalogenations?
the major product will be the more highly substituted alkene (the more dtable alkene)
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What is the mechanism for an E2 reaction?
- EtO- removes a beta proton; C-H breaks; new pi bod forms and Br begins to depart
- Partial bonds in the transition state: C-H and C-Br bonds break, new pi C-C bond forms
- C=C is fully formed and the other products are EtOH and Br-
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What is Hofmann's Rule?
Most elimination reactions follow Zaitsev's rule in which the ost stable alkenes are the major products. However, under some circumstances, the major elimination product is the less substituted, less stable alkene
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Stereochemistry of E2 reactions
- The 5 atoms involved in the transition state of an E2 reaction (including the base) must lie in the same plane
- The anti coplanar conformation is the preferred transition state geometry
- The anti coplanar transition state is staggered (and therefore of lower energy), while the syn coplanar transition state is eclipsed
- Orientation requirement- H and Br have to be anti periplanar (trans-coplanar)
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E2 elimination where there are two axial beta hydrogens
Both Ha and Hb hydrogens are anti t the chlorine in this, the more stable conformation
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What is dehydration?
Loss of a molecule of water
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What happens when an alcohol is dehydrated?
It forms an alkene when heated with a strong acid
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What do the temperature and concentration of acid required to dehydrate an alcohol depend on?
Depends on the structure of the alcohol substrate
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What is the most difficult alcohol to dehydrate?
Primary alcohols
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What is the easiest alcohol to dehydrate?
Tertiary alcohol
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What happens to some primary and secondary alcohols during dehydration?
Can undergo rearrangements of their carbon skeletons during dehydration
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