9 hemija predavanje
DESCRIPTION
predavanja iz hemije sa rudarsko - geoloskog fakultetaTRANSCRIPT
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HIBRIDIZACIJA ORBITALA I GEOMETRIJA MOLEKULA
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H
CH H
H
C – H veze iste dužine iste energije
Ugao veze
109o 28’ )
Kako objasniti?
↑↓ ↑ ↑
6C 1s2 2s2 2p2
hibridizacijaAO
Kvantno-mehanička teorija
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Sta se očekuje na osnovu elektronske konfiguracije C-atoma?
C (1s2 2s22px12py
1) – trebalo bi očekivati da gradi dve kovalentne veze. To se ne dešava.
Atom C najpre dovođenjem energije prelazi u pobuđeno stanje (jedan elektron se rasparuje i iz 2s2 prelazi u 2pz orbitalu. PROBLEM ! Kako nastaju četiri energetski ekvivalentne orbitale?
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Hibridizacija - matematičko kombinovanje atomskih orbitala istih ili sličnih energija
Hibridizacija atomskih orbitala ne postoji u izolovanom atomu – to je samo kvantno-mehanički model
Nastale hibridne orbitale (degenerisane orbitale)
Broj degenerisanih orbitala jednak je broju atomskih orbitala
Razlikuju se od atomskih orbitala čijom kombinacijom nastaju
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Ee
2s2p
osnovno stanje
ekscitacija
2s2p
sp3
Eh
pobudjeno stanje hibridne
orbitale
sp3 - hibridizacija
↑↓ ↑ ↑ 6C 1s2 2s2 2p2
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Hibridizacijom (mešanjem) atomskih orbitala dobijaju se četiri energeski ekvivalentne orbitale.
U ovom slučaju(CH4) kombinuju se jedna s i tri p orbitale i nastaju četiri sp3 orbitale – tetraedarska hibridizacija (svaka od četiri sp3 orbitale sadrži po jedan elektron i usmerena je prema roglju tetraedra)
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4 sp3 hibridne orbitale
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geometrija - tertaedar 4 σ veze nastaju preklapanjem s-sp3 4 σ (s-sp3)
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NH3 - tertaedar
N
HH
H
) (HNH) = 107o
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NH3 - tertaedar
N
HH
H
2s2p
osnovno stanje
Eh
2s
sp3
hibridne orbitale
) (HNH) = 107o
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geometrija - tertaedar
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) (HOH) = 105o
sp3
O
H H
sp3
1s1s
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H2O - tertaedar
H O
H O
HH
2s2p
Eh
sp3
osnovno stanje
hibridne orbitale
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Primeri sp3 hibridizacije : NH3 i H2O
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Primer: sp3 hibridizacija, etan C2H6 (postoje dva C atoma – 2 centralna atoma). Ukupno 7 σ veza: 2 sp3 orbitale C atoma čine jednu C-C σ vezu i, C-H 6σ veza
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sp2 - hibridizacija
BF3
5B 1s2 2s2 2p1
Ee
osnovno stanje
ekscitacija Eh
pobudjeno stanje
hibridne orbitale
2s2p 2s
2p 2pxsp2
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Bsp2
sp2
sp2
F
F
F
120o
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sp hibridizacija BeCl2
BeCl2
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VIŠESTRUKA VEZA
C Cσπ
C C
σ - primarna veza π - sekundarna veza
Da bi nastala π –veza atomi moraju imati nehibridizovane p - orbitale Višestruke veze (dvostruke i trostruke) grade samo atomi koji kod kojih je nakon hibridizacije ( sp2 ili sp) ostalo nehibridizovanih p - orbitala
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DVOGUBA (DVOSTRUKA) VEZA
Ee
2s2p
osnovno stanje
ekscitacija
2s2p
sp2pz
Eh
pobudjeno stanje hibridne orbitale
C2H4 eten (etilen)
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1σ (sp2 –sp2) 4σ (sp2 – s) 1 π (pz –pz)
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TROGUBA (TROSTRUKA) VEZA
C2H2 etin (acetilen)
Ee
2s2p
osnovno stanje
ekscitacija
2s2p
sppzpy
Eh
pobudjeno stanje hibridne orbitale
C C HH180o
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1σ (sp –sp) 2σ (sp – s) 1 π (pz –pz) 1 π (py –py)
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C = C 134 pm
C ≡ C 121 pm
C – C 154 pm
Dužina veze
Jačina veze - energija koju je potrebno dovesti da bi se veza raskinula
C – C 346 kJ/mol
C = C 602 kJ/mol
C ≡ C 835 kJ/mol
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Molekuli imaju odredjenu strukturu i geometrijski oblik zbog usmerenosti kovalentne
veze
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REZONANCIJA
Predstavlja strukture a ne različite vrste molekula Predstavlja jedan te isti molekul čija se struktura prikazuje na više načina Rezonancija (mezomerija) nije pojava to je metoda
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• Teorija valentne veze elektronski par σ-veze ili π-veze lokalizovan
je između dva određena atoma i predstavlja lokalizovanu molekulsku orbitalu
• Teorija molekulskih orbitala molekulske orbitale pružaju se preko celog
molekule i mogu se smatrati delokalizovanim, jer povezuju više atoma
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Strukture mnogih molekula i jona moguće je prikazati (opisati) pomoću lokalizovanih orbitala
Strukture nekih molekula i jona mogu se
objasniti samo ako predpostavimo da se elektroni nalaze u delokalizovanim MO
Teorija valentne veze takve strukture tumači REZONANCIJOM ili MEZOMERIJOM
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Leverova pravila: -Kako su rasporedjeni valentni elektroni u kovalentnim molekulima -Poštuje se oktetno pravilo
nv = h + 4a - (ne/2) broj vezujućih elektronskih parova
broj slobodnih elektronskih parova
ns = ne - h - 4a
h – broj vodonikovih atoma a – broj atoma težih od vodonika ne – ukupan broj valentnih elektrona
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• Elektronska strukturna formula CO2 prema pravilu okteta može se pisati na 3 načina
nv = 0 + 4x3 - (16/2) = 4
ns = 16 - 0 - 4x3 = 4
CO O CO OCO O
CO115 ppm
CO
izmereno rastojanje
122 ppm CO
110 ppm
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Stvarna struktura CO2 – rezonantni hibrid kanonskih struktura
C OO CO O CO O
C OO
CO O
CO O
Rezonantni hibrid stabilizovan je energijom rezonancije
Energija rezonantne strukture niža je od energije bilo koje pojedinačne strukture rezonantnog hibrida
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Benzen (C6H6)
Delokalizacija π - elektrona
C – atomi su sp2 hibridizovani
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Ozon
OO
O
OO
O
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1,3-butadien, H2C=CH-CH=CH2
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• Teorija molekulskih orbitala lako tumači postojanje delokalizovanih elektrona u molekulama, jer prema toj teoriji elektroni pripadaju celom molekulu
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Oblici molekula Prema teoriji odbijanja slobodnih elektrona (VSEPR valence-shell electron-pair repulsion theory) geometrija svakog molekula zavisi od rasporeda elektronskih parova u spoljašnjem sloju.
Elektronski parovi koji se nalaze u valentnim orbitalama i slobodni elektronski parovi uvek teže da se uređeju tako da obrazuju strukturu molekula sa najmanjom odbojnom silom (najmanji sadržaj energije).
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Primeri:
Molekul BeCl2- linerani molekul
Najmanje odbijanje dva para zajedničkih (valentnih )elektrona je onda ako se po jedan par elektrona nalazi sa suprotnih strana Be atoma (ugao od 180°)
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Molekul BF3 - trigonalno planaran (trougaoni). Ovakav raspored daje mogućnost da se tri zajednička elektronska para najbolje uređuju sa najmanjim odbijanjem .
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Molekul SnCl2- ima jedan slobodan elektronski par . Molekul nije linearan ,već savijen. Odbijanje koje potiče od slobodnog elektronskog para smanjuje ugao (nije 180°)
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Molekuli : CH4, NH3, H2O
Tetraedar
Zajedničko im je da imaju po četiri elektronska para u valentnom sloju centralnog atoma koji su upravljeni prema temenima tetraedra.
Razlika je u položaju atoma u molekulu.
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NH3 - nepravilna trostrana piramida (zbog slobodnog eletronskog para) Obratiti pažnju na veličinu ugla.
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Molekul H2O- nepravilna trostrana piramida(savijen oblik) (dva slobodna elektronska para se odbijaju a deluju i na elektrone u vezivnim orbitalama, što uslovljava smanjenje vrednosti ugla).
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Molekul PCl5 - trigonalna bipiramida (oko centralnog atoma se nalazi 5 elektronskih parova)
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Molekul SF4- trigonalna bipiramida
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Molekul SF6- oktaedar
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