Referat Chimia Organica
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INTRODUCERE
Chimia Organica este ramura chimiei in care sunt studiati compusii
carbonului si reactiile lor. O mare varietate de clase de substante, cum
ar fi medicamente, materiale plastice, fibre naturale si sintetice, ca
si carbohidrati, proteine si grasimi sunt formate din molecule organice.
Chimistii organici determina structura moleculelor organice, studiaza
reactiile lor si elaboreaza proceduri de sinteza a compusilor organici.
Chimia organica a avut un profund efect in secolul 20:a imbogatit
materialele naturale si a sintetizat materiale artificiale care au
imbunatatit medicina , au crescut comfortul oamenilor.
Inceputul chimiei organice este asociat cu descoperirea, in 1828, de
catre chimistul german Friedrich Wöhler, ca substanta inorganica
cianura de amoniu poate fi convertita in laborator in uree, o substanta
organica gasita in urina animalelor. Inainte de aceasta descoperire
chimistii credeau ca interventia asa numitei "forte de viata" este
necesara pentru a sintetiza substante organice. Experimentul lui Wöhler
a darmat bariera dintre substantele organice si anorgance. Chimistii
moderni considera compusii organici a fi cei care contin carbon si un
alt sau mai multe elemente, cel mai des hidrogenul, oxigenul, azotul,
sulful si halogenii, dar cateodata si alte substante.
LEGATURI SI FORMULE ORGANICE
Formula moleculara a unui compus indica numarul fiecarui fel de atomi
in molecula acelei substante. Fructoza, sau sucul de struguri(C6H12O6),
consta in molecule continand 6 atomi de carbon, 12 atomi de hidrogen si
6 atomi de oxigen. Pentru ca sunt cel putin 15 alti compusi care au
aceeasi formula chimica, pentru a distinge o molecula de alt, este
folosita o formula structurala pentru a arata aranjamentul spatial al
atomilor.
Chiar si anliza care ne da procentajul de carbon, oxigen si hidrogen
nu pot distinge C6H12O6 de riboza, C5H10O5, un alt zahar in care
proportiile elementelor sunt aceleasi, adica 1:2:1.
Fortele care tin atomii uniti intr-o molecula sunt legaturile
chimice, cae sunt de trei tipuri: ionice, covalente si metalice.
Legaturile ionice sunt determinate de atractia sarcinilor de sens opus.
Legaturile covalente sunt formate de perechi de electroni comuni.
Experimentul lui Wöhler(fig.1), de exemplu a rezultat in schimbarea
din legaturi ionice in cianura de amoniu in legaturi covalente din uree.
In cianura de amoniu, atractia dintre un grup de 5 atomi NH4+ purtand
sarcina pozitiva si un grup de alti 3 atomi in CNO-, purtand sarcina
negativa constituie o legatura ionica. in gruparea dinnge from ionic
bonds in ammonium cyanate to covalent bonds in urea. In gruparea NH4+
,cle 4 linii -N spre H reprezinta legaturi covalente dar si ionice. La
fel , in gruparea CNO-, in molecula de uree reprezinta legaturi
covalente. Inmcalzirea cianurii de amoniu are ca rezultat rearanjarea
legaturilor. Proprietatea carbonului de a forma legaturi covalente nu
este unica. Legaturile dintre azot si hidrogen sunt si ele
covalente.Totusi abilitatea acestuia de a forma legaturi covalente cu
alti atomi de carbon in lanturi sau inele, il distinge de alte elemente.
Alte elemente nu formeaza lanturi mai mari de 8 atomi identici. Aceasta
proprietate a carbonului, si faptul ca, carbonul aproape totdeauna
formeaza 4 legaturi cu alti atomi conteaza pentru un numar foarte mare
de compusi cunoscuti. Cel putin 80% din cele 5 milioane de compusi
chimici, cunoscuti inca de la inceputul anilor 80 contin carbon.
CLASIFICARI SI NOMENCLATURI
Consecintele proprietatii unice a carbonului se manifesta in cea mai
simpla clasa de substante organice, hidrocarburile.
A Alcanii
Copmusul-parinte al acestei familii este metanul, CH4. Urmatorii
membri ai familiri sunt etanul (C2H6), propanul (C3H8) si butanul
(C4H10), asa ca formula generala pentru fiecare membrual familiei este
CnH2n+2.Pentru compusi care contin mai mult de 4 atomi, sunt utilizate
prefixe grecesti cu sufixul - ane la numele compusilor (pentan, hexan,
octan, s.a.m.d).
Numele de pentan, butan, etc totusi nu sugereaza singure structura
moleculara. De exemplu, din formula moleculara C4H10 se pot intelege
doua formule structurale diferite.Compusii cu aceeasi formule
moleculare, dar formule structurale diferite sunt numiti izomeri.In
cazul butanului, nume comune de izomeri sunt n-butan si izobutan.Ureea
si cianura de amoniu sunt si ei izomeri: sunt izomeri structurali ai
formulei moleculare CH4 N2O.
Formula C8H18 are 18 izomeri si C20H42 are366319 izomeri
teoretici.Uniunea Internationala a Chimiei Pure si Aplicate (IUPAC) a
cazut de acord in 1890 la un sistem de nomenclatura si l-a revizuit ca
sa incorporeze noi descoperiri.
In sistemul de nomenclatura IUPAC cel mai lung lant de carbon este
numarat pentru a da lanturilor laterale cea mai mica suma. Cele 3
lanturi laterale din fig. 4 sunt atomii de carbon 2,2 si 4; daca lantul
ar fi numarat in directia opusa, lanturile laterale ar fi 2,4 si 4.
Atunci, numele de trimetilpentan ar fi corect, pt ce din el rezulta cea
mai mica suma.
Alta familie de hidrocarburi, ciclanii, au un inel sau o structura
ciclica; cel mai mic inel contine 3 atomi de carboni.Ciclanii au formula
chimica generala CnH2n.
B Alchenele si Alcanii
Familia alchenelor, sau izomerii ciclanilor si alcanilor, cum mai
este numita, este reprezentata de formula generala CnH2n.Aceasta familie
de hidrocarburi este caracterizata de una sau mai multe legaturi duble
intre atomii de carbon.Propanul si ciclopropanul, de exemplu, sunt
izomeri, cum sunt 1,3-dimetilciclohexanul si
3,4-dimetil-2-hexanul.Legaturile duble se intampla in compusii ciclici.
Chimistii folosesc de obicei o notatie prescurtata cand scriu
formulele structurale ai compusilor ciclici organici.Fiecare atom are,
de la sine inteles 2,1 sau 0 atomi de hidrogen in legatura.
Alcanii sau acetilenele sunt a treia mare familie a hidrocarburilor si
au formula generala CnH2n-2 si contin mai putini atomi de hidrogen decat
alchenele
C Grupuri Functionale
Alti atomi, cum ar fi la clor, oxigen sau azot pot fi inlocuiti cu
hidrogenul in alcani.
D Izomeri Optici si Geometrici
Natura tetraedica a formelor legaturilor carbonului dicteaza niste
proprietatiai compusilor organici. Cand 4 grupuri diferite de atomi sunt
atasate la un atom central de carbon , 2 molecule diferite pot fi
construite in spatiu. De exemplu, compusul acid lactic exista in doua
forme , un fenomen numit izomerism optic.Izomerii optici sunt legati in
acelasi fal cum on obiect si imaginea sa din oglinda sunt legate. O
Oglinda plasata intre cele doua structuri ar reflecta ca gruparile: CH3
din primul reflectand pozitia CH3-ului din cealalta, OH reflectand OH,
si tot asa.
Optical isomers have exactly the same chemical properties and all of the
same physical properties except one: the direction that each type of
isomer turns a plane of polarized light (see Optics). Dextro-lactic
acid, or D-lactic acid, turns the plane of polarized light to the right
and levo-lactic acid, or L-lactic acid, to the left. Racemic lactic acid
(a 1:1 mixture of D- and L-lactic acid that is found in sour milk)
exhibits zero rotation because left and right rotations cancel each
other.
Double bonds in carbon compounds give rise to geometric isomerism (not
related to optical isomerism) if each double bond has different groups
attached. A molecule of 2-heptene, for example, may be arranged two ways
in space because rotation about the double bond is restricted. When the
like groups, hydrogen atoms in this case, are on opposite sides of the
double bonded carbon atoms, the isomer is called trans and when the
hydrogens are on the same side, the isomer is called cis.
E Saturation Compounds containing double or triple bonds are said to
be unsaturated. Unsaturated compounds can undergo addition reactions
with various reagents that cause the double or triple bonds to be
replaced with single bonds. Addition reactions cause unsaturated
compounds to become saturated. Although saturated compounds are
generally more stable than unsaturated compounds, two double bonds in
the same molecule cause less instability if they are separated by a
single bond. Isoprene, the building block for natural rubber, has this
so-called conjugated structure, as does retinal, a compound derived from
vitamin A.
Complete conjugation in a six-membered carbon ring has a more profound
effect, a stabilizing influence so strong that the compound is no longer
unsaturated. Benzene, C6H6, and the family of cyclic compounds called
aromatic hydrocarbons, do not add the reagents that react with isoprene,
alkanes, and alkenes. In fact, the properties of aromatic compounds are
so different that a more appropriate symbol for benzene is the hexagon
on the extreme right of Fig. 13 rather than the other two. The circle
inside the hexagon suggests that the six electrons represented as three
conjugated double bonds belong to the entire hexagon and not to
individual carbons at the corners of the hexagon. Other aromatic
compounds are shown in Fig. 14.
Cyclic molecules may contain atoms of elements other than carbon. The
most common so-called hetero atoms are sulfur (S), nitrogen (N), and
oxygen (O), although others-for example, boron (B), phosphorus (P), and
selenium (Se)-are known.
IV SOURCES OF ORGANIC COMPOUNDS
Coal tar was once the only source of aromatic and some heterocyclic
compounds. Petroleum was the source of aliphatic compounds that contain
such substances as gasoline, kerosene, and lubricating oil. Natural gas
supplied methane and ethane. These three categories of natural
substances are still the major sources of organic compounds for most
countries. When petroleum is not available, however, a chemical industry
can be based on acetylene, which in turn can be synthesized from
limestone and coal. During World War II, Germany was forced into just
that position when it was cut off from reliable petroleum and
natural-gas sources.
Table sugar from cane or beets is the most abundant pure chemical from a
plant source. Other major substances derived from plants include
carbohydrates such as starch and cellulose, alkaloids, caffeine, and
amino acids. Animals feed on plants and other animals to synthesize
amino acids, proteins, fats, and carbohydrates.
V DETERMINATION OF STRUCTURE
The use of chemical reactions to identify the structures of organic
compounds has been replaced largely by instrumental methods since 1940.
Infrared spectra are used to identify functional groups, and ultraviolet
spectroscopy can distinguish aromaticity and certain kinds of
unsaturation in a molecule. A nuclear magnetic resonance (nmr) spectrum
gives the largest amount of information about the structure of a
compound; infrared and ultraviolet spectra complement rather than
duplicate such data. Proton resonance spectroscopy is sometimes used to
determine the nature of the local environment of the hydrogen atoms in a
molecule and it can often simultaneously supply the ratios of types of
hydrogen. More recently, carbon-13 nuclear magnetic resonance
spectroscopy has been used to derive complementary information to the
proton data. Also, an X-ray spectrum may be necessary to determine
three-dimensional aspects of structure in a complex organic molecule.
See Chemical Analysis.
VI PHYSICAL PROPERTIES OF ORGANIC COMPOUNDS In general, covalent
organic compounds are distinguished from inorganic salts by low melting
points and boiling points. The ionic compound sodium chloride (NaCl),
for example, melts at about 800° C (about 1470° F), but the strictly
covalent molecule, carbon tetrachloride (CCl4), boils at 76.7° C (170°
F). Between these temperatures an arbitrary line may be drawn at about
300° C (about 570° F) to distinguish most covalent from most ionic
compounds. A large fraction of organic compounds melt or boil below
300° C, although exceptions exist. Organic compounds generally dissolve
in nonpolar solvents (liquids that do not have localized electric
charges) such as gasoline or carbon tetrachloride, or solvents of low
polarity such as alcohols, acetic acid, and acetone. Organic compounds
are often insoluble in water, a strongly polar solvent.
Hydrocarbons have low densities, often about 0.8 compared to water, 1.0,
but functional groups may increase the densities of organic compounds to
1.0. Only a few organic compounds have densities greater than 1.2,
generally those containing multiple halogen atoms.
Functional groups capable of forming hydrogen bonds generally increase
viscosity (resistance to flow) in molecules. For example, the
viscosities of ethanol, ethylene glycol, and glycerol increase in that
order. These compounds contain one, two, and three OH groups,
respectively, which form strong hydrogen bonds.
VII CHEMICAL REACTIONS Chemists ordinarily design organic reactions to
be carried out at optimum conditions for the particular reaction in
order to produce maximum yields. To do so necessarily means that the
chemist must be aware of desirable catalysts, whether or not the
reaction is reversible, and how to take advantage of equilibrium
positions. As an example, two different products may be obtained in
sulfonating naphthalene (adding the SO3H functional group to a
naphthalene molecule) by taking advantage of the reversibility of this
reaction (Fig. 16).
At a temperature of 80° C (176° F) the rate of reaction at the
á-position is faster than the rate at the â-position. Consequently, a
91 percent yield of á-naphthalenesulfonic acid can be obtained at 80°;
at a higher temperature, the â-isomer predominates. At 160° C (320°
F) the á-isomer desulfonates more rapidly than the â-form, so an 85
percent yield of â-naphthalenesulfonic acid, the more stable isomer, is
obtained.
Catalysts are frequently essential for rapid chemical reactions. Water,
for example, will not add to unsaturated compounds unless a small amount
of a strong acid (represented in Fig. 17 by H+) is present. See Acids
and Bases; Catalysis; Chemical Reaction.
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