The carbonyl group, viz., >C=O in an organic or inorganic compound, decides the property of the compound depending on its surrounding.Carbonyl group, in addition to -OH group, in form of an aldehyde or ketone is an important functional group in carbohydrates.
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CARBON MONOXIDE, CO
Carbon monoxide molecule looks like an isolated carbonyl group. It is the simplest oxocarbon isoelectronic with the cyanide ion & molecular nitrogen. It is a neutral oxide. Carbon monoxide is a strong ligand forming several metal complexes called metal carbonyls.On the other hand, carbon-dioxide is acidic and is a weak ligand.
MOLECULAR PROPERTIES
The bond length of carbon monoxide is 112.8 pm (1pm=10-12m), consistent with a triple bond, as in molecular nitrogen (N2). The boiling point (82 K) and melting point (68 K) are very similar to N2 (77 K and 63 K, respectively). The bond dissociation energy of CO (1072 kJ mol-1) is greater than that of N2 (942 kJ mol-1).
The dipole moment of CO is 0.122 D. The carbon monoxide as a ligand, reverses the polarity of the dipole with a net negative charge on the oxygen.
BIOLOGICAL EFFECTS
Carbon monoxide causes adverse effects in humans by combining with iron in hemoglobin forming a complex, carboxy-hemoglobin in the blood. This prevents hemoglobin from carrying oxygen to the tissues.
Carboxy-hemoglobin can revert to hemoglobin, but the recovery takes time because the complex is fairly stable.
Carbon monoxide, like cyanide CN¯ ion, is poisonous as it combines(coordinates) with iron in hemoglobin as mentioned above .It may also be noted that both the poisonous species CO & CN¯ are iso-electronic with each other, though the reverse need not be true as in the case of nitrogen molecule.
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CARBON DIOXIDE, CO2 ; O=C=O
Please refer Article 5-(GC-5) in this Blog(Click below)
5-(GC-5):https://dradchem.wordpress.com/2015/03/12/gc-5-the-vital-compound-for-life-carbon-dioxide-co2/
Interestingly, the CO2 molecule is like two carbon monoxide molecules & looking like combination of two carbonyl groups
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PHOSGENE
Phosgene,COCl2 is a planar molecule. The C=O bond distance: 1.18 Å (1Å=10-10m) , C—Cl distance: 1.74 Å and the Cl—C—Cl bond angle is 111.8°. It is one of the simplest acid chlorides, being formally derived from carbonic acid, H2CO3.
SIMILAR COMPOUNDS: Carbonyl fluoride, COF2. Bromophosgene, COBr2
DIPHOSGENE, is ClCO2CCl3 ,colorless liquid valuable reagent in the synthesis of organic compounds. Diphosgene is related to phosgene and has comparable toxicity, but is more conveniently handled because it is a liquid, whereas phosgene is a gas.
TRIPHOSGENE: Bbis(trichloromethyl)carbonate (BTC), C3Cl6O3 is used as a safer substitute for phosgene, because at room temperature it is a solid crystal, as opposed to phosgene which is a gas. Triphosgene crystals decompose above 200°C.
It may be noted that phosgene type of compounds do not contain phosphorus as in the case of phosphine, PH3.
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CARBONIC ACID, H2CO3 :
Please refer Article 5-(GC-5) in this Blog
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ALDEHYDES, RCHO
An aldehyde or alkanal is an organic compound containing a formyl group,-CHO as the functional group.
Carbon, in an organic carbonyl compound, like aldehydes & ketone is sp2-hybridized. The planar carbon center is connected by a double bond to oxygen and a single bond to an alkyl group & hydrogen in the case of aldehyde . The C-H bond is not acidic.
Because of resonance stabilization of the conjugate base, not only the aldehydic hydrogen, even the α-hydrogen (-CH2CHO) in an aldehyde is more acidic, with a pKa near 17, than a C-H bond in a typical alkane (pKa about 50). This is attributed to (i) the electron-withdrawing quality of the formyl center and (ii) the conjugate base, an enolate anion, delocalizes its negative charge.
Aldehydes (except those without hydrogen on the alpha carbon, such as formaldehyde and benzaldehyde) having α-hydrogen exist in either the keto or the enol tautomer and undergo aldol condensation, giving an aldol, a product which contains aldehyde & alcohol .
Aldehydes (those without hydrogen on the alpha carbon, such as formaldehyde and benzaldehyde) not having α-hydrogen undergo Cannizaro’s reaction (self oxidation & reduction). Two molecules of such an aldehyde react to produce a primary alcohol and a carboxylic acid using a hydroxide base
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KETONES, RCOR
Ketones are more acidic (pKa ≈ 20) than alkanes (pKa ≈ 50). The carbonyl group in ketones and aldehydes show an IR spectrum near 1700 cm−1. The 13C NMR spectra gives downfield peak at 200 ppm.
Aldehydes and ketone undergo some common reactions characteristic of carbonyl compounds.
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ACTIVE METHYLENE GROUP, –CO-CH2-CO-
Malonic acid, HOOC-CH2-COOH; acetyl acetone, CH3COCH2COCH3 and similar compounds & their derivatives have methylene group -CH2-, which is very active called active methylene group: -CO-CH2-CO-. Active methylene group is more acidic and reactive due to two electron withdrawing nature of the two carbonyl groups on both sides of the methylene group,viz.,-CH2-. Such kind of compounds undergo reactions like Knoevenagel reaction, Michael condensation etc.
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AMIDES ,RCONH2 & RCONHR
Amides, RCONH2 and substituted amides, R-CONH-R, are neutral whereas amines, RNH2 are basic. The electron withdrawing nature of the carbonyl group makes the lone pair of electrons on amide-nitrogen not to be available to behave as base.Thus the carbonyl group plays a vital role in deciding the nature of the -NH- group (basic , neutral & acidic nature of amines , amides & imides, respectively). Although amides contain carbonyl group >C=O , they do not undergo reactions characteristic of carbonyl compounds.
The amide linkage is rigid and resists hydrolysis. Nylons are polyamides, hard and resistant to chemical reactions. Amide linkages in proteins, in biochemical context are called peptide bonds, -NH-CO-.
Low molecular weight amides, such as dimethylformamide {HCON(CH3)2}, are common solvents. Many drugs are amides, including penicillin and LSD. Plant N-alkylamides have a wide range of biological functions.
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IMINDES, -CO-NH-CO-
Amides are neutral whereas imides are acidic. The electron withdrawing nature of the two carbonyl groups on both sides of -NH- plays a vital role in deciding the acidic nature of imides as in the case of -CO-CH2-CO-. Imides are derived from dicarboxylic acids-succinimide and phthalimide.
Many hard and electrically conducting polymers contain imide sub-units as polyimides. Kapton consists of two imide groups derived from aromatic tetracarboxylic acids.
Interest in the bioactivity of imide-containing compounds was initiated by the discovery of the bioactivity of the cycloheximides as an inhibitor of protein biosynthesis in certain organisms. Number of fungicides and herbicides contain the imide functionality like captan.
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CARBOXYLIC ACIDS, RCOOH
Carboxylic acids are proton (H+) donors and are the most common type of organic acids. Carboxylic acids are weak acids, as they are partially dissociated into H+ and RCOO− in aqueous solution. The carbonyl group, attached to the –OH group, makes the –OH group in -COOH acidic.
Although amides, imides and carboxylic acids contain carbonyl group >C=O, they do not undergo reactions characteristic of carbonyl compounds.
In conclusion, one may say that carbonyl group plays vital role in several chemical compounds and in bio-molecules such as proteins as amide (peptide) linkage, carbohydrates as aldoses and ketoses, fats as esters, and in simple compounds like CO2 as the one of the main compound in photosynthesis, CO though toxic, may have some additional role in the field of chemistry.
Please go back to CONTENTS for all the Articles in PoC:
https://dradchem.wordpress.com/2015/06/22/properties-of-chemistry-poc-contents-2/