duminică, 7 aprilie 2013

HIDROCARBURI ACICLICE NESATURATE

Hidrocarburi aciclice

Def:  Se numesc alchine hidrocarburi aciclice nesaturate care contin in molecula lor  o tripla legatura. Intre 2 at. de C si in care raportul si in care raportul intre nr. de at. de C si H este dat de formula CnH2n-2 in care n=nr. de at. de C din molecula.
Caracteristici:  Alchinele sunt : -hidrocarburi
                                        -aciclice
                                        -nesaturate (NE=2, datorita prez. a 2                  
                                                              leg.π in componenta leg. triple)

Nomenclatura. Serie Omoloaga.
Dand lui n. Val. Intregi obtinem termenii seriei omoloage in care ca in orice serie omoloaga , 2 termeniu consecutivi  se diferentieaza intre ei printr-o gr. Mtilen. (-CH2-)
Denumirea alchinelor se formeaza prin inlocuirea suf. –an de la alcanul corespunzator , cu suf. –ina . Seria omoloaga incepe de la n=2


n
CnH2n-2
Alcan
CnH2n-2
Alchina
1
CH4
Metan
-
-
2
C2H6
Etan
C2H2
Etina (acetilena)
3
C3H8
Propan
C3H4
Propina
4
C4H10
butan
C4H6
butina


Radicalii alchinici:
CH=CH        - etilena                          CH≡ C        -etinil                    
CH3-C≡C    - propinil                        CH2-C≡CH - propagil

Izomeria la alchine.
Alchinele sunt izomerii de functiuni cu:  -alcanii diciclici (Spirali)
                                            -cicloalchine
                                            -diene
Alchinele , de la n=4  prez. fen. de iz. de pozitie dat posibilitatii pe care are tripla leg. de a ocupa poz. diferite in catena
       Butina prez. urmatorii 2 izomeri de pozitie
C4H8:       CH≡C-CH2-CH3           1 butina
               CH3-C≡C-CH3                   2 butina

       Structura alchinelor 
In catena alchinelor se intalnesc 2 tipuri de at. de C .
▪ 2 at. de C hibridizati sp - cei doi at. de C implicati in form. tiplei leg.
▪ at. de C hibridizati sp3 – implicati in formarea leg. simple
Acetilena   -primul termen al seriei omoloage prezinta o structura (simetrie diagonala) dat hidrocarburii de simetrie digenala sp a celor 2 at. de C . Datorita hibridizarii sp unghiul si distanta dintre cei 2 at. de C triplu legati=1,21Å fata de:
                                                       -1,54 Å  in leg, C-C    C≡C  1,21Å
                                                       -1,33 Å in leg, C=C      <180˚
Tot datorita hibridizarii scade si distanta C-H  de la 1,1 in cazul Csp3-H la 0,6 in Csp-H                H-C≡C-H  0,6 Å
Consecinta discreta a hibridizarii sp a at. de C este si polarizarea leg. C-H mai accentuata decat la celelalte leg.
Leg Csp – H= leg polara cu S- pe Csp si  S+ pe H. Csp-H .  Aceasta polarizare a leg. confera acetilenei , respectiv alchinelor cu tripla o le. marginala un caracter slab acid!!

Obtinerea alchinelor
I  metode industriale
       1. Din metan:   -Prin cracare in arc electric
                                 -Prin ardere incompleta
        2. Din carbura de Ca  (carbit)  CaC2
II     metode de laborator
          1 Dubla eliminare de hidracid din derivati dihalogenati geminali sau vicinali
          2 Det. alchinelor sup.  prin alchilarea acetilenelor metalice 

I   1. Obtinerea din metan
       La temperaturi ridicate 1500˚C => transferul metanului in acetilena ˚
                                       2CH4—>C2H2+3H2
       Industrial transformarea se face prin doua procedee diferite intre ele prin sursa de energie necesara reactiei
       Se identifica :
a.             procedeul de cracare a CH4 in arc electric
b.             procedeul arderii incomlete

a) In cazul acestui procedeu energia necesara reactiei esste furnizata de descompunerea elementelor si se produc intre cei 2 electrozi metalici alimentati la o sursa de curent continu. Alaturi de reactia principal;a au loc si o serie de reactii secundare si de ceea ptr stoparea lor are loc stropirea brusca a mediului de reactie cu un jet de apa rece. Nu poate fi totusi evitata reactia de formare a C liber                             
                                                                       1500˚C
                                       CH4  — —>  C+2H2
               Procedeul se aplica la Borzesti
       b)In cazul acestui procedeu metanul introdus in reactor este utilizt pe de o parte ptr obtinerea acetilenei , iar pe de alta parte ptr furnizarea energiei termice prin combustie. Si in acest caz au loc reactii secundare dintre care cea mai importanta este reactia de obtinere a gazului de sinteza
                               CH4+1/2O2—>CO+2H2
         2.Obtinerea acetilenei din carbon de Ca
       Carbura de Ca=compus ionic Ca 2+ si C2 2-  Ionul C2  este format din 2 at. de Chidrocarborizati sp uniti printr-o tripla leg. si la care gasim cate o sarcina negativa HC≡CH
In CaC2cele 2 sarcini pozitive de la ionul carbura au fost neutralizati de Ca 2+
                    
Industrial CaC2 (carbid) se obtine prin reducerea la 2500 ˚ C cu cox metalurgic a oxidului de Ca obtinut prin descompunerea termica a calcarului
                      
                        800-1000˚ C
         CaCO3  ------------------->               CaO + CO2
            (calcar)
                        2500˚ C        
         CaO + 3C ---------------->       CaC2 + CO                                                                                                                                                
                                                         (carbid)

             CaC2 fiind o carbura ionica (acetilura) a unui metal s hidrolizeaza in conditii obisnuite cu degajarea acetilenei. Reactia se aplica atat la scara mica in gen. De acetilena in cazul sudurii oxiacetilenice cat si la scara industriala.
            Reactia are loc violent si rapid
        CaC2 +2H2O--------------> Ca(OH)2 +C2H2 
                                                                   (acetilena)
                                                       generatorul de acetilena


Metale de laborator

1.    Eliminarea de hidracid din derivatii dihalogenati
a.         vicinali:
Transformarea se produce in prezenta de KOH/alc la temperatura de 100-150˚ C. In prima etapa ( I ) are loc eliminarea hidracidului obtinut deriv. halog. , care in a doaua ( II ) etapa la temperatura mai mare de 150˚ C elimina hidracidul cu transformare in alchina corespunzatoare.

                             KOH alc                             KOH alc
-HC-CH-  ----------------> -C=CH-   ----------->    -C≡C-
    X X         100-150˚ C        X             t > 150˚ C       
                       - HX                              -HX

                          1,2-diclor-etan
                               KOH alc                             KOH alc
                                CH2- CH2  ------------> CH=CH2  ------------------>  HC≡CH
                          Cl       Cl       100-150˚ C   Cl               t > 150˚ C
                                                            - HCl                              -HCl

!!!         Alchenele nu se deshidrogeneaza la alchine. Transformarea unei alchene in alchina se realizeaza printr-o succesiune de reactii respectiv:
KOHalc
>C=C<+Br2 -----—>  >C -C<  -----—>-C=C<  ----—>   -C≡C- ----—> CH2=CH2 ------—> CH≡CH
Br     Br


                                           100+50˚                               -HBr
CH2=CH2 +2Br--—>CH2-CH2------> CH=CH2 -----------> CH≡CH
                          Br        Br        -HBr      Br                      t>150˚C

                                                                           KOH alc                                                       KOH alc
CH3-CH=CH+Br2----------->CH3-CH-CH2-------------->CH3-CH=CH2----------------->CH3-C≡CH
                                                          Br    Br       100-150˚  -HBr                              Br               t>150˚-HBr

              

                            CH2=CH2                   C≡CH
                                       
                                       ----------->


                         Stiren             Fenil acetilena
                   
                                                 Br      Br
                              CH=CH2+Br2        CH-CH2         C≡CH
                                                                                  KOH alc
                                     ----------->          ----------->      
                                                        -2HBr


               b)Dubla dehidrogenare a unui derivat dehalogenat geminal
Reactia are loc in prezenta de KOH solutie alcolica cu obtinerea in etapa I monohalogenurei ele vinil corespund , iar in etapa aII-a cu alchinei
                                                          x               -Hx                                          -HX
-C-CH2------------>  -C=CH- -----------> -C≡C-
                                                                                      x                   KOH alc              x                     KOH alc

Derivatii dihalogenati vacinali se obtin in urma reactiei unei grupari crbonil in pentahalogenura de P
                                                                x
                      !   C=O=PX5----------> C
                                                                      -pox3                x
                    carbonilici gr. Carbonil
          Astfel din alchida acetica se poate obtine acetilena

                                                                    Cl                        Cl                                                      KOH alc
          CH3-CH=O+PCl5---------->CH3-CH3 ---------->CH3-CH---------->CH2=CH---------->CH≡CH
                                                               -POCl3                       Cl                                     Cl                                                        -HCl
oxiclorura de fosfor

                       CH3                                               CH3   Cl     
                    C=O+PCl5--------->     C     ------------->CH=CH--------->CH3-C≡CH
                   CH3                      -POCl3         CH 3   Cl           -HCl             CH3    Cl


                                                                      CH3-CH2
C=O--------->CH3-C≡C-CH3
                                                                              CH3


                                                                                                                                                                                          
  CH3-CH2                                                CH3-CH2      Cl
             C =O+PCl5c--------->           C   ---------> CH3-CH=C-CH3---------> CH3-C≡C-CH3
           CH3                                                          CH3     Cl                                                 Cl                  -HCl


2.    Obtinerea alchinelor superioare prin alchilarea cu conp halogenati (vezi pe larg alc. acetilenei , ionice la subcapitolul reactii de substitutie la Csp) Alchinele cu tripla legatura marginala reactiicu Na metalice la 150˚C printr-o reactie de substitutie a H de Csp marginal cu obtinerea unei acetiluri monoacide. Aceasta acetilura poate reactiona cu un derivat halogenat avand loc subst. Na si formarea unei alchine superioare
                                      150˚ C                      +                                           +   +H-R
-C≡CH+Na--------->-C≡C ־ --------->-C≡C ־ --------->-C≡C-R 
                                  1/2H2                                                                                   -Nax

       Astfel din acetilena se poate obt. prin monoalchinarea orice alchina cu tripla marginala marginala , iar prin dialchinare o alchina cu tripla nemarginala
                                   
                                    100˚ C              _     +     +X-R
CH≡CH+Na --------->CH≡C  Na  --------->-CH≡C-R
                               -1/2H2         +Na                            -NaX            Imp ptr ca se lungeste catena , introdus C
                                                        (200 ˚ C)-1/2H2  
                                                            + _        _     +    
                           NaC≡CNa    
                                                               
                               +2R’-X
                                                                --------> R’-C≡C-R’
-2Nax            Imp ptr ca se obtine tripla la mijloc  

  
                CH4-------->CH≡C-CH3

            1500˚ C                                  +Na                                         +CH3-Cl
                     2CH4--------->CH≡CH------------------->CH≡CNa--------->CH≡C-CH3
                                          -3H2                                 -150˚ C-1/2H2                               -NaCl


                                        CH4--------->CH3-C≡C-CH3

                                       
                                                    1500˚ C                                    Na                                         +Na                                  +2CH3-Cl    
               2CH4--------->CH≡CH---------> CH≡CNa---------> NaC≡CNa----------> CH3-C≡C-CH
                                          -3H2                      1500˚ C-1/2H2                   200˚ C-H2-H2                             -2NaCl                               



                                                  
                                        CH4--------->CH3-CH-C≡ C-CH-CH3
                                                                                                                 Acetilena


            1500˚ C                       +Na                            +Na                                                                                
2CH4--------->CH≡CH--------->CH≡CNa--------->NaCl≡CNa--------->CH3-CH-Cl+NaC≡CNa+Cl-CH-CH3
       -3H2                              150˚ C                                 -NaCl                                                                     CH3                                 CH3                                                                                                      
                                                                                          --------->CH3-CH-C≡C-CH-CH3
                                                                  -NaCl                CH3               CH3