8.3: Vifungo vingi

Malengo ya kujifunza

• Eleza bonding nyingi covalent katika suala la atomiki orbital mwingiliano
• Kuhusiana na dhana ya resonance kwa π-bonding na electron delanication

Mfano wa orbital wa mseto unaonekana kuhesabu vizuri kwa jiometri ya molekuli inayohusisha vifungo moja vya covalent. Je, pia ina uwezo wa kuelezea molekuli zenye vifungo viwili na vitatu? Tayari tumejadili kwamba vifungo vingi vinajumuisha vifungo vya σ na π. Next tunaweza kufikiria jinsi sisi taswira vipengele hivi na jinsi yanahusiana na orbitals mseto. Muundo wa Lewis wa ethene, C ₂ H ₄, unatuonyesha kwamba kila atomu ya kaboni imezungukwa na atomi nyingine moja ya kaboni na atomi mbili za hidrojeni.

Kielelezo\(\PageIndex{2}\)).

Muundo wa Lewis unaonyeshwa ambapo atomi mbili za kaboni zinaunganishwa pamoja na dhamana mbili. Kila atomu ya kaboni huunganishwa na atomi mbili za hidrojeni kwa dhamana moja.

Kielelezo\(\PageIndex{1}\): Katika ethene, kila atomi ya kaboni ni sp ² iliyochanganywa, na orbitals sp ² na p orbital ni moja kwa moja ulichukua. Orbitals ya mseto huingiliana ili kuunda vifungo vya σ, wakati orbitals p kwenye kila atomi ya kaboni huingiliana ili kuunda dhamana π.

Mchoro unaonyeshwa katika sehemu mbili, zilizounganishwa na mshale unaoelekea kulia unaoitwa, “Uchanganyiko.” Mchoro wa kushoto unaonyesha mshale unaoelekea juu ulioitwa, “E.” Kwa upande wa chini wa mshale ni mstari mfupi, usio na usawa unaoitwa, “2 s,” ambao una mishale miwili ya wima inakabiliwa juu na chini. Kwa upande wa juu wa mshale ni mfululizo wa mistari mitatu mifupi, ya usawa iliyoandikwa, “2 p.” Juu ya seti zote mbili za mistari ni maneno, “Orbitals katika atomi C pekee.” Mstari miwili ina mishale ya wima, inayowakabili inayotolewa juu yao. Upande wa kulia wa mchoro unaonyesha mistari mitatu mifupi, ya usawa iliyowekwa nusu hadi nafasi na kila kinachoitwa, “s p superscript 2.” Mshale wa nusu unaoelekea juu unatolewa kwa wima kwenye kila mstari. Juu ya mistari hii ni mstari mfupi mfupi, usio na usawa, ulioandikwa, “p.” Zaidi ya seti zote mbili za mistari ni maneno, “Orbitals katika s p superscript 2 mahuluti C atomi katika C subscript 2 H subscript 4.”

Dhamana π katika dhamana ya C =C mara mbili inatokana na mwingiliano wa tatu (iliyobaki) 2 p orbital kwenye kila atomu ya kaboni ambayo haihusiki katika mahuluti. Hii unhybridized p orbital (lobes inavyoonekana katika nyekundu na bluu katika Kielelezo\(\PageIndex{2}\)) ni perpendicular kwa ndege ya orbitals sp ² mseto. Kwa hiyo, orbitals 2 p unhybridized huingiliana kwa mtindo wa upande mmoja, juu na chini ya mhimili wa internuclear na kuunda dhamana π.

Kielelezo\(\PageIndex{2}\): Katika molekuli ya ethene, C ₂ H _4, kuna (a) vifungo tano σ. Moja C—C σ matokeo ya dhamana kutoka mwingiliano wa sp ² orbitals mseto juu ya atomi kaboni na moja sp ² mseto orbital juu ya atomi nyingine kaboni. Nne C—H vifungo kutokana na mwingiliano kati ya C atomi 'sp ² orbitals na orbitals s juu ya atomi hidrojeni. (b) Dhamana ya π hutengenezwa na kuingiliana kwa upande kwa upande wa orbitals mbili ambazo hazijaharibika katika atomi mbili za kaboni. Vipande viwili vya dhamana π viko juu na chini ya ndege ya mfumo wa σ.

Michoro mbili zinaonyeshwa kinachoitwa, “a” na “b.” Mchoro a inaonyesha atomi mbili za kaboni na orbitals tatu za rangi ya zambarau za puto zilizopangwa katika ndege karibu nao na orbitals mbili nyekundu za puto zilizopangwa kwa wima na perpendicularly kwa ndege. Kuna mwingiliano wa orbitali mbili za zambarau kati ya atomi mbili za kaboni, na zingine orbitali nne za zambarau ambazo zinakabiliwa na nje ya molekuli zinaonyeshwa kuingiliana na orbitali za bluu za mviringo kutoka atomi nne za hidrojeni. Mchoro b inaonyesha picha sawa na mchoro a, lakini orbitals nyekundu, wima ni kuingiliana juu na chini ya ndege ya molekuli kuunda maeneo mawili lebo, “One pi dhamana.”

Katika molekuli ya ethene, atomi nne za hidrojeni na atomi mbili za kaboni zote ziko kwenye ndege moja. Kama ndege mbili za orbitals sp ² mseto tilted jamaa na kila mmoja, orbitals p bila kuwa oriented kuingiliana kwa ufanisi kujenga π dhamana. Configuration planar kwa molekuli ethene hutokea kwa sababu ni imara zaidi bonding mpangilio. Hii ni tofauti kubwa kati ya vifungo σ na π; mzunguko kuzunguka vifungo moja (σ) hutokea kwa urahisi kwa sababu mwingiliano wa mwisho hadi mwisho wa orbital hautegemei mwelekeo wa jamaa wa orbitali kwenye kila atomu katika dhamana. Kwa maneno mengine, mzunguko kuzunguka mhimili wa internuclear haubadili kiwango ambacho orbitals za kuunganisha σ huingiliana kwa sababu wiani wa elektroni wa bonding ni ulinganifu kuhusu mhimili. Mzunguko kuhusu mhimili wa internuclear ni ngumu zaidi kwa vifungo vingi; hata hivyo, hii ingebadilika sana kuingiliana kwa mhimili wa mbali wa mzunguko wa π, kimsingi kuvunja dhamana π.

Mchoro wa atomi ya kaboni na orbitals mbili za rangi ya zambarau kama zambarau zilizoandikwa, “sp” iliyopangwa kwa mtindo wa mstari unaozunguka unaonyeshwa. Orbitals nne nyekundu kama balloon ni iliyokaa katika jozi katika shoka y na z kuzunguka kaboni na ni lebo, “unhybridized p orbital,” na, “Pili unhybridized p orbital.”

Kielelezo\(\PageIndex{3}\): Diagram of the two linear sp hybrid orbitals of a carbon atom, which lie in a straight line, and the two unhybridized p orbitals at perpendicular angles.

In molecules with sp hybrid orbitals, two unhybridized p orbitals remain on the atom (Figure \(\PageIndex{3}\)). We find this situation in acetylene, H−C≡C−H, which is a linear molecule. The sp hybrid orbitals of the two carbon atoms overlap end to end to form a σ bond between the carbon atoms (Figure \(\PageIndex{4}\)). The remaining sp orbitals form σ bonds with hydrogen atoms. The two unhybridized p orbitals per carbon are positioned such that they overlap side by side and, hence, form two π bonds. The two carbon atoms of acetylene are thus bound together by one σ bond and two π bonds, giving a triple bond.

Figure \(\PageIndex{4}\): (a) In the acetylene molecule, C₂H_2, there are two C–H σ bonds and a C≡C triple bond involving one C–C σ bond and two C–C π bonds. The dashed lines, each connecting two lobes, indicate the side-by-side overlap of the four unhybridized p orbitals. (b) This shows the overall outline of the bonds in C₂H₂. The two lobes of each of the π bonds are positioned across from each other around the line of the C–C σ bond.

Two diagrams are shown and labeled, “a” and “b.” Diagram a shows two carbon atoms with two purple balloon-like orbitals arranged in a plane around each of them, and four red balloon-like orbitals arranged along the y and z axes perpendicular to the plane of the molecule. There is an overlap of two of the purple orbitals in between the two carbon atoms. The other two purple orbitals that face the outside of the molecule are shown interacting with spherical blue orbitals from two hydrogen atoms. Diagram b depicts a similar image to diagram a, but the red, vertical orbitals are interacting above and below and to the front and back of the plane of the molecule to form two areas labeled, “One pi bond,” and, “Second pi bond,” each respectively.

Hybridization involves only σ bonds, lone pairs of electrons, and single unpaired electrons (radicals). Structures that account for these features describe the correct hybridization of the atoms. However, many structures also include resonance forms. Remember that resonance forms occur when various arrangements of π bonds are possible. Since the arrangement of π bonds involves only the unhybridized orbitals, resonance does not influence the assignment of hybridization.

For example, molecule benzene has two resonance forms (Figure \(\PageIndex{5}\)). We can use either of these forms to determine that each of the carbon atoms is bonded to three other atoms with no lone pairs, so the correct hybridization is sp². The electrons in the unhybridized p orbitals form π bonds. Neither resonance structure completely describes the electrons in the π bonds. They are not located in one position or the other, but in reality are delocalized throughout the ring. Valence bond theory does not easily address delocalization. Bonding in molecules with resonance forms is better described by molecular orbital theory.

Mchoro unaonyeshwa kuwa imeundwa na miundo miwili ya Lewis iliyounganishwa na mshale uliomalizika mara mbili. Picha ya kushoto inaonyesha atomi sita za kaboni zilizounganishwa pamoja na vifungo viwili na vya moja ili kuunda pete ya upande mmoja. Kila kaboni pia huunganishwa na atomu ya hidrojeni kwa dhamana moja. Picha sahihi inaonyesha muundo huo, lakini vifungo mara mbili na moja kati ya atomi za kaboni vimebadilisha nafasi.

Kielelezo\(\PageIndex{5}\): Kila atomi ya kaboni katika benzini, C _{6 H 6}, ni sp ² iliyosababishwa, kwa kujitegemea fomu ya resonance inachukuliwa. Elektroni katika vifungo vya π hazipo katika seti moja ya orbitals p au nyingine, lakini badala ya kufutwa katika molekuli.

Mfano\(\PageIndex{1}\): Assignment of Hybridization Involving Resonance

Baadhi ya mvua ya asidi hutokana na mmenyuko wa dioksidi ya sulfuri na mvuke wa maji ya anga, ikifuatiwa na malezi ya asidi ya sulfuriki. Dioksidi ya sulfuri\(\ce{SO2}\),, ni sehemu kubwa ya gesi za volkeno pamoja na bidhaa ya mwako wa makaa ya mawe yenye sulfuri. Je, ni hybridization ya\(S\) atomi ndani\(\ce{SO2}\)?

Suluhisho

Miundo ya resonance ya\(\ce{SO2}\) ni

 

Miundo miwili ya Lewis iliyounganishwa na mshale wa kumalizika mara mbili huonyeshwa. Muundo wa kushoto unaonyesha atomi ya sulfuri yenye jozi moja pekee ya elektroni na ishara chanya ambayo ni moja iliyofungwa upande mmoja hadi atomu ya oksijeni yenye jozi tatu za elektroni na ishara hasi. Atomu ya sulfuri inaunganishwa mara mbili upande mwingine kwa atomi nyingine ya oksijeni yenye jozi mbili za elektroni. Muundo wa mkono wa kulia ni sawa na wa kushoto isipokuwa kwamba nafasi ya atomi ya oksijeni iliyofungwa mara mbili inabadilishwa. Katika miundo miwili atomi za oksijeni zilizounganishwa huunda angle ya papo hapo kwa suala la atomi ya sulfuri.

Atomi ya sulfuri imezungukwa na vifungo viwili na jozi moja ya elektroni katika muundo wowote wa resonance. Kwa hiyo, jiometri ya jozi ya elektroni ni mpango wa trigonal, na uharibifu wa atomi ya sulfuri ni sp ².

Zoezi\(\PageIndex{1}\)

Another acid in acid rain is nitric acid, HNO₃, which is produced by the reaction of nitrogen dioxide, NO₂, with atmospheric water vapor. What is the hybridization of the nitrogen atom in NO₂? (Note: the lone electron on nitrogen occupies a hybridized orbital just as a lone pair would.)

Answer

sp²

Summary

Multiple bonds consist of a σ bond located along the axis between two atoms and one or two π bonds. The σ bonds are usually formed by the overlap of hybridized atomic orbitals, while the π bonds are formed by the side-by-side overlap of unhybridized orbitals. Resonance occurs when there are multiple unhybridized orbitals with the appropriate alignment to overlap, so the placement of π bonds can vary.