Lecture 6 Flashcards
(16 cards)
Substitution reactions
Substitution reaction of alkanes occurs by the replacement of hydrogen atoms with more reactive atoms such as chlorine.
Substitution reactions of alkanes
Radical — molecular fragment with an odd number of shared electrons (at least one unpaired electrons), very reactive
Free radical are like robbers (which are deficient in energy)
Free radicals attack and snatch energy from others to satisfy their octet structure.
Alkenes
At least one double bond between carbons
General Formula CnH2n
Rules of naming alkenes
- Identify functional group (double bond)
- Identify the longest “C” chain that contains the functional group
- Number the chain so the functional group has the lowest number.
What to do if more than one double bond is present
If more than one double bond is present, indicator their positions by using the number of the first carbon of each bond and use the suffix-diene (for 2 D.B.), -triene (for 3 D.B.), -tetraene (for 4 D.B.), etc.
We want the double bonds to have the lowest numbers.
Isomerism across Double Bond
Double bond in alkenes are rigid and holds attached groups in fixed positions (cis trans isomers possible)
CIS — means on the same side, (of D.B.), Cis-isomers
TRANS — means across (of D.B.), Trans-isomers
How to name cis trans isomers
The prefixes: cis or trans are place infront of the alkene’s names (when there are cis or trans isomers)
Geometrical Isomers/ StereoIsomers
- The bond in alkenes does not permit the rotation, thus all atoms attached directly to the carbon double bond lin in plane (sp^2)
- Such compounds are different in physical and chemical properties as well as their geometry
- These isomers have the same connectivity of atoms and differ only in the arrangement of atoms in space.
- Cis-trans isomers result from restriction rotation
Usually trans isomers are more stable than cis isomers, Less Steric Hindrance
Properties of Alkenes
- Carbon double bond in alkenes change the physical properties of alkenes
- At room temperature, alkenes exists in all three phases: solid, liquid and gases
- B.p. and m.p. are similar to those in alkanes with he same number of carbon
- Cis isomers have a lower melting point than trans
- Solubility: insoluble in water but dissolve in organic solvents.
Synthesis of Alkenes
- Elimination reaction of alkyl halides using a strong base - Dehydrohalogenation
- Dehydration of alcohols
- Catalytic cracking of alkanes
Reactions of alkenes
4 basic types in OrgChem
1. Addition
2. Elimination
3. Substitution
4. Rearrangement
Alkene substitution
Alkenes names as substituents are called ALKENYL groups.
Can be names systematically as ethenyl, properly etc. or by common names methylene, vinyl, allyl, etc.
Alkynes
At least on triple carbon bond
General formula CnH2n-2
Naming Alkynes
Alkynes names are similar as alkenes
All the usual rules apply except the “yne” is used
Double and triple bonds
What happens when double and triple bonds appear in the same molecule?
- Alkenes and alkynes are considered to have equal priority
- In an molecule with both a double and triple bond, whichever is closer tot he end of the chain determines the direction of numbering
- In the case where each would have the same position numbers the double bond takes the lower number
- In the name, “ene” come before “yne” because of alphbetization
Physical properties of alkynes
- They resemble those hydrocarbon of similar shape and molecular weight
- They have low melting points and boiling points
- MP and BP increase as the number of carbon increases
- They are soluble in organic solvent and insoluble in water
