ch04 alkenes and alkynes
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WILLIAM H. BROWN
THOMAS POON
www.wiley.com/college/brown
C H A P T E R F O U R
Alkenes and Alkynes
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Unsaturated Hydrocarbons
• Unsaturated Hydrocarbon: Contans one o!
"o!e #a!$on%#a!$on do&$le o! t!'le $onds o!
$en(ene%lke !n)s.
* Alkene: #ontans a #a!$on%#a!$on do&$le $ond
and +as t+e )ene!al ,o!"&la CnH-n. * Alkyne: #ontans a #a!$on%#a!$on t!'le $ond
and +as t+e )ene!al ,o!"&la CnH-n%-.
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Unsaturated Hydrocarbons
• Arene: $en(ene and ts de!ates /C+ 01
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Benzene and Phenyl Groups
• We do not st&dy $en(ene and ts de!ates
&ntl C+a'te! 0.
* Ho2ee!3 2e s+o2 st!t&!al ,o!"&las o,
#o"'o&nds #ontann) a '+enyl )!o&' $e,o!e t+at
t"e. * T+e '+enyl )!o&' s not !ea#te &nde! any o, t+e
#ondtons 2e des#!$e n #+a'te!s 4%5.
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Structure of Alkenes
• T+e t2o #a!$on ato"s o, a do&$le $ond and
t+e ,o&! ato"s $onded to t+e" le n a 'lane3
2t+ $ond an)les o, a''!o6"ately 7-89.
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Structure of Alkenes
• A##o!dn) to t+e o!$tal oe!la' "odel3 a
do&$le $ond #onssts o, one σ $ond ,o!"ed
$y oe!la' o, sp- +y$!d o!$tals and one π
$ond ,o!"ed $y oe!la' o, 'a!allel - p
o!$tals.• Rotating by 90°breaks the pi bond.
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Cis-Trans Isomerism
• Be#a&se o, !est!#ted !otaton a$o&t a C*C
do&$le $ond3 )!o&'s on ada#ent #a!$ons a!e
et+e! cis o! trans to ea#+ ot+e!. * Be#a&se o, non$onded nte!a#ton st!an $et2een
alkyl s&$stt&ents o, t+e sa"e sde o, t+e do&$le$ond3 a trans alkene s "o!e sta$le t+an an
so"e!# cis alkene.
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Structure of Alkynes
• T+e ,&n#tonal )!o&' o, an alkyne s a
#a!$on%#a!$on t!'le $ond.
• A t!'le $ond #onssts o,:
* One σ $ond ,o!"ed $y t+e oe!la' o, sp +y$!d
o!$tals. * T2o π $onds ,o!"ed $y t+e oe!la' o, sets o,
'a!allel - p o!$tals.
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Nomenclature of Alkenes
• IUPAC No"en#lat&!e o, alkenes
* Use t+e n,6 -en- to s+o2 t+e '!esen#e o, a
#a!$on%#a!$on do&$le $ond.
* N&"$e! t+e 'a!ent #+an to )e t+e 7st #a!$on o,
t+e do&$le $ond t+e lo2e! n&"$e!. * Follo2 IUPAC !&les ,o! n&"$e!n) and na"n)
s&$stt&ents.
* Fo! a #y#loalkene3 t+e n&"$e!n) o, t+e ato"s o,
t+e !n) t+e "&st $e)n 2t+ t+e t2o #a!$ons o,
t+e do&$le $ond.
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Nomenclature of Alkenes
• IUPAC no"en#lat&!e o, alkenes
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-He!ene
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Nomenclature of Alkenes
• So"e alkenes3 'a!t#&la!ly lo2%"ole#&la!%
2e)+t ones3 a!e kno2n al"ost e6#l&sely $y
t+e! #o""on na"es.
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"onfi#uration$ Cis-Trans
• %he cis-trans system: Con,)&!aton s
dete!"ned $y t+e o!entaton o, ato"s o, t+e"an #+an.
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"onfi#uration$ &'(
• To Ass)n an E3; #on,)&!aton3 ,!st ass)n a '!o!ty
to t+e s&$stt&ents on ea#+ #a!$on o, t+e do&$le$ond. * I, t+e )!o&'s o, +)+e! '!o!ty a!e on t+e sa"e sde o, t+e
do&$le $ond3 t+e #on,)&!aton s Z /
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"onfi#uration$ &'(
• P!o!ty !&les
7. P!o!ty s $ased on ato"# n&"$e!= t+e +)+e! t+e ato"# n&"$e!3t+e +)+e! t+e '!o!ty.
-. I, '!o!ty #annot $e ass)ned on t+e $ass o, t+e ato"s $onded
d!e#tly to t+e do&$le $ond3 look to t+e ne6t set o, ato"s= '!o!ty s
ass)ned at t+e ,!st 'ont o, d,,e!en#e.
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"onfi#uration - &'(
>. Ato"s 'a!t#'atn) n a do&$le o! t!'le $ond a!e
#onsde!ed to $e $onded to an e?&alentn&"$e! o, s"la! ato"s $y sn)le $onds.
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"onfi#uration - &'(
• &!ample: Na"e ea#+ alkene $y t+e E,Z
syste" and s'e#,y ts #on,)&!aton.
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Cis-Trans Isomerism
* T+e #on,)&!aton o, t+e do&$le $ond n
#y#lo'!o'ene t+!o&)+ #y#lo+e'tene "&st $e cis.T+ese !n)s a!e not la!)e eno&)+ to
a##o""odate a t!ans do&$le $ond.
* Cy#loo#tene s t+e s"allest #y#loalkene t+at #an
a##o""odate a trans do&$le $ond.
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Cis-Trans Isomerism
• @enes3 t!enes3 and 'olyenes
* Fo! an alkene 2t+ n #a!$on%#a!$on do&$le $onds3 ea#+o, 2+#+ #an s+o2 cis-trans so"e!s"3 )n cis-trans
so"e!s a!e 'oss$le.
* Consde! -3%+e'tadene= t +as ,o&! cis-trans so"e!s3
t2o o, 2+#+ a!e d!a2n +e!e.
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Cis-Trans Isomerism
* ta"n A +as ,e C%C do&$le $onds.
* Fo&! o, t+e ,e #an s+o2 cis-trans so"e!s".
* ta"n A s t+e all%trans so"e!.
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Physical Properties
• Alkenes and alkynes a!e non'ola! #o"'o&nds.
* T+e only att!a#te ,o!#es $et2een t+e! "ole#&les
a!e dispersion forces.
• T+e '+ys#al '!o'e!tes o, alkenes and alkynes
a!e s"la! to t+ose o, alkanes o, s"la! #a!$onskeletons.
* T+ose t+at a!e l?&d at !oo" te"'e!at&!e a!e less
dense t+an 2ate! /7.8 )"L1.
* T+ey dssole n ea#+ ot+e! and n non'ola! o!)an#
solents.
* T+ey a!e nsol&$le n 2ate!.
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%he Acidity of %erminal Alkynes
• One o, t+e "ao! d,,e!en#es $et2een t+e
#+e"st!y o, alkynes3 and alkanes and alkeness t+at t+e +yd!o)en ato" o, a te!"nal alkyne s
s&,,#ently a#d# /'K a -41 t+at t an $e !e"oed
$y a st!on) $ase s+ as sod&" a"de3 NaNH-3to )e an alkyne anon.
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%he Acidity of %erminal Alkynes
• Be#a&se 2ate! /'K a 74.D1 s a st!on)e! a#d
t+an a#etylene o! a te!"nal alkyne3
+yd!o6de on s not a st!on) eno&)+ $ase ,o!
an a#d%$ase !ea#ton 2t+ a te!"nal alkyne.
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%he Acidity of %erminal Alkynes
• T+e 'K a al&es ,o! alkane and alkene
+yd!o)ens a!e so la!)e t+at no $ase s st!on)
eno&)+ to !e"oe a +yd!o)en ,!o" et+e! o,
t+ese #lasses o, #o"'o&nds.
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%he Acidity of %erminal Alkynes
• Ho2 do 2e a##o&nt ,o! t+e a#dty o, a te!"nal alkyne
#o"'a!ed to an alkane o! alkene
• To ans2e! t+s ?&eston3 2e "&st ,o#&s on t+e sta$lty
o, t+e anon de!ed ,!o" ea#+ #lass o, +yd!o#a!$on.
• T+e lone 'a! o, ele#t!ons o, an alkane anon les n ansp> +y$!d o!$tal3 2+#+ +as -4 s #+a!a#te!.
• Fo! an alkene anon3 t+e lone 'a! les n an sp- +y$!d
o!$tal3 2+#+ +as >> s #+a!a#te!.
• Fo! a te!"nal alkyne anon3 t+e lone 'a! les n an sp
+y$!d o!$tal3 2+#+ +as 48 s #+a!a#te!.
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%he Acidity of %erminal Alkynes
• T+e )!eate! t+e s #+a!a#te! o, t+e o!$tal
$ea!n) t+e ne)ate #+a!)e3 t+e )!eate! t+esta$lty o, t+e anon3 and t+&s t+e )!eate!
a#d# t+e +yd!o)en !e"oed.
• O, t+e se!es o, #o"'o&nds alkane3 alkene3and alkyne3 t+e #a!$on o, a te!"nal alkyne s
t+e "ost ele#t!one)ate /48 s #+a!a#te!1.
T+e!e,o!e3 a te!"nal alkyne anon s t+e"ost sta$le o, t+e se!es3 and a te!"nal
alkyne s t+e "ost a#d#.
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Ho* to determine the Inde! of Hydro#en +eficiency ,IH+
Bass a!e t+e Sat&!ated HCs /Alkanes1 CnH-nG-
E6 CH
CH7
−
C H
5
-
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4 douleonds! d & 1 ring
2d & 2 rings1 d & !rings2 tripleonds
H-
C C CH CH C CH-
HC C CH-
CH-
C CH
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Alkenes and Alkynes
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"nd Ch#pter 4