organic chemistry alkynes
TRANSCRIPT
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Organic Chemistry
Alkynes
by
Dr. Seema Zareen & Dr. Izan Izwan Misnon
Faculty Industrial Science & Technology
[email protected] & [email protected]
For updated version, please click on
http://ocw.ump.edu.my
AlkynesBy Seema Zareenhttp://ocw.ump.edu.my/course/view.php?id=152
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Expected Outcomes
In the end of this chapter, student will have the
ability to:
- Describe characteristics and physical properties
of alkyne
- Draw and name alkyne using IUPAC
nomenclature
- Write alkyne reaction mechanism
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Contents
• Alkynes
• Physical properties
• Nomenclature
• Alkyne reaction mechanisms
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Alkynes
• Alkynes contain a carbon—carbon triple bond.
• Terminal alkynes have the triple bond at the end of the carbon chainso that a hydrogen atom is directly bonded to a carbon atom of thetriple bond.
• Internal alkynes have a carbon atom bonded to each carbon atom ofthe triple bond.
• An alkyne has the general molecular formula CnH2n-2, giving it fourfewer hydrogens than the maximum possible for the number ofcarbons present. Thus, the triple bond introduces two degrees ofunsaturation.
Introduction—Structure and Bonding
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• Recall that the triple bond consists of 2 bonds and 1 bond.
• Each carbon is sp hybridized with a linear geometry and bond angles of1800.
Introduction—Structure and Bonding
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• Alkynes are named in the same general way that alkenes are named.
• In the IUPAC system, change the –ane ending of the parent alkane name tothe suffix –yne.
• Choose the longest continuous chain that contains both atoms of the triplebond and number the chain to give the triple bond the lower number.
• Compounds with two triple bonds are named as diynes, those with threeare named as triynes and so forth.
• Compounds containing both a double and triple bond are named as enynes.The chain is numbered to give the first site of unsaturation (either C=C orCC) the lower number.
• The simplest alkyne, H-CC-H, named in the IUPAC system as ethyne, ismore often called acetylene, its common name.
• The two-carbon alkyl group derived from acetylene is called an ethynylgroup.
Nomenclature
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Naming Priority of functional groups
• Carboxylic acids
• Esters
• Aldehydes
• Ketones
• Alcohols
• Alkenes • Alkynes• Alkanes , Ethers , Halides
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NomenclatureExamples of Alkynes nomenclature
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Nomenclature
Examples of alkyne nomenclature
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Common Names
Named as substituted acetylene.
CH3 C CH methylacetylene
CH3 CH
CH3
CH2 C C CH
CH3
CH3
isobutylisopropylacetylene
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Name these:
CH3 CH
CH3
CH2 C C CH
CH3
CH3
CH3 C C CH2 CH2 Br
CH3 C CH
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Name these:
CH3 CH
CH3
CH2 C C CH
CH3
CH3
CH3 C C CH2 CH2 Br
CH3 C CHpropyne or 1-propyne
5-bromo-2-pentyne
2,6-dimethyl-3-heptyne
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CH2 CH CH2 CH
CH3
C CH
CH3 C C CH2 CH
OH
CH3
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CH2 CH CH2 CH
CH3
C CH
4-methyl-1-hexen-5-yne
CH3 C C CH2 CH
OH
CH3
4-hexyn-2-ol
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• The physical properties of alkynes resemble those of
hydrocarbons of similar shape and molecular weight.
• Alkynes have low melting points and boiling points.
• Melting point and boiling point increase as the number of
carbons increases.
• Alkynes are soluble in organic solvents and insoluble in
water.
• Up to 4 carbons, gases at room temperature.
Physical Properties
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• Recall that alkynes are prepared by elimination
reactions. A strong base removes two equivalents of
HX from a vicinal or geminal dihalide to yield an alkyne
through two successive E2 elimination reactions.
Preparation of Alkynes
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• Like alkenes, alkynes undergo addition reactions because they contain relatively weak bonds.
• Two sequential reactions can take place: addition of one equivalent of reagent forms analkene, which can then add a second equivalent of reagent to yield a product having fournew bonds.
Introduction to Alkyne Reactions—Additions
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Introduction to Alkyne Reactions—Additions
addition of hydrogen halides
addition of halogens
addition of water
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• Two equivalents of HX are usually used: addition of one mole forms a vinyl halide, whichthen reacts with a second mole of HX to form a geminal dihalide.
Hydrohalogenation—Electrophilic Addition of HX
• Alkynes undergo Hydrohalogenation, i.e the addition of hydrogen halides, HX.
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Hydrohalogenation—Electrophilic Addition of HX
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Hydrohalogenation—Electrophilic Addition of HX
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• Electrophilic addition of HX to alkynes is slower thanelectrophilic addition of HX to alkenes, even though alkynesare more polarizable and have more loosely held electronsthan alkenes.
• Markovnikov addition in step [3] places the H on the terminal carbon toform the more substituted carbocation A, rather than the less substitutedcarbocation B.
NOT
A Bmore stable carbocation
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• Halogens X2 (X = Cl or Br) add to alkynes just as they do to alkenes. Addition of one moleof X2 forms a trans dihalide, which can then react with a second mole of X2 to yield atetrahalide.
Halogenation—Addition of Halogen
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Halogenation—Addition of Halogen
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• In the presence of strong acid or Hg2+ catalyst, the elements of H2Oadd to the triple bond, but the initial addition product, an enol, isunstable and rearranges to a product containing a carbonylgroup—that is, a C=O. A carbonyl compound having two alkylgroups bonded to the C=O carbon is called a ketone.
Hydration—Electrophilic Addition of Water
less stable enolketone
H2O has been added
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• Internal alkynes undergo hydration with concentrated acid, whereas terminal alkynesrequire the presence of an additional Hg2+ catalyst—usually HgSO4—to yield methylketones by Markovnikov addition of water.
Hydration—Electrophilic Addition of Water
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• Consider the conversion of a general enol A to the carbonyl compound B. A andB are tautomer: A is the enol form and B is the keto form of the tautomer.
Hydration—Electrophilic Addition of Water
• Equilibrium favors the keto form largely because the C=O is much stronger thana C=C. Tautomerization, the process of converting one tautomer into another, iscatalyzed by both acid and base.
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Hydration—Electrophilic Addition of Water
Protonation and deprotonation
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Hydration—Electrophilic Addition of Water
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CALCULATING
DEGREE OF UNSATURATION
CcHhNnOoXx
DBU = (2c + 2) – (h – n + x)2
• 1o unsaturation = 1 C=C or 1 ring
• 2o unsaturation = 2 C=C, 2 rings, or CΞC, or combination of C=C & rings
• 3o unsaturation = combination of double bonds, triple bonds, rings
• 4o unsaturation = typically indicates an aromatic ring
DBU = C- H/2 +1
OR
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Summary
The degree of unsaturation can be calculated readily from the
molecular formula of all compounds containing carbon, hydrogen,
oxygen, nitrogen, sulphur, or the halogens, by applying the following
rules:
Rule 1: Replace all halogens in the molecular formula by hydrogens.
Rule 2: Omit oxygen and sulphurs.
Rule 3: For each nitrogen, omit the nitrogen and one hydrogen.