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Acidity and Basicity of Organic Compounds

We know that an acidic compound can donate hydrogen ion while a basic compound can give hydroxyl ion. The number of H+ and OH- ion from a chemical compound can represent with the help of their acidity or basicity. Let’s discuss what is acidity and basicity? The number of hydrogen ion from the compound is called as basicity of that acid while the number of hydroxyl ion from a base is called as acidity of respective compound. 

The acidity or basicity of a certain element depends upon the position of that element in the periodic table. For simple organic compounds like alkane (ethane), amines (methylamine) and alcohol (methanol) the concept of acidity and basicity is entirely dependent on the molecular structure of the molecule. For example the acidity of given organic compounds increases from alkane to alcohols. On the contrary the basic nature of their conjugated bases increases in the opposite direction as given below;

Stronger Acid

As we move from carbon to oxygen via nitrogen, their nature can be observed with the help of their conjugate base as for the more stable the conjugate base, the strength of the acid also increases. There is negative charge on these elements in their conjugated base like negative charge on C in ethyl anion and negative charge in N in methylamine anion and on O in methoxide anion. We know that the electro negativity of elements increases from C to O and a more electronegative atom can better bear a negative charge compare to less electronegative element. 
Hence the methoxide anion would be the most stable and least basic) and ethyl anion is the least stable and most basic. Similar concept can be applicable on water and ammonia. Out of these two compounds; ammonia is more basic compare to water as O-atom is more electronegative and tightly hold the lone pair of electrons compare to N-atom in ammonia. Hence N-atom more tends to form bond through its lone pair and becomes more basic. From top to bottom in the periodic table the concept of acidity can be observed which we can apply on the basic nature of halides. 
From fluoride to iodide the basic nature decrease which increases the acidic nature of hydrohalic acid from HF to HI. The concept of acidic and basic nature of organic compounds is very useful in the study of biological organic chemistry such as this trend proves that thiols are more acidic compare to alcohols.  The periodic trend of acidity in the periodic table through periods and groups can be written as given below;  

Periodic Trends in Acidity

The resonance effect is also helpful in the determination of acidity and basicity of groups which have exchangeable proton (H+) bound to different elements. Let’s take an example of this effect. 
The resonance in ethanol and acetic acid can be used to determine the acidic and basic nature of these organic compounds. After losing H+ ion; RCOOH converts into RCOO- ion which is stabilised due to equal distribution of negative charge over both oxygen atoms. That is the reason; RCOOH can easily convert into RCOO- ion as RCOOH is less stable compare to RCOO-. On the contrary; alcohols (ROH) can be stabilised after losing H+ ion as they form RO- (alkoxide ion) which is less stable due to positive inductive effect of alkyl group.

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From Wikipedia

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solidchemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon such as diamond and graphite, are considered inorganic. The distinction between "organic" and "inorganic" carbon compounds, while "useful in organizing the vast subject of chemistry... is somewhat arbitrary".

Organic chemistry is the science concerned with all aspects of organic compounds. Organic synthesis is the methodology of their preparation.



The name "organic" is historical, dating back to the 1st century. For many centuries, Western alchemists believed in vitalism. This is the theory that certain compounds could only be synthesized from their classical elements — Earth, Water, Air and Fire — by action of a "life-force" (vis vitalis) possessed only by organisms. Vitalism taught that these "organic" compounds were fundamentally different from the "inorganic" compounds that could be obtained from the elements by chemical manipulation.

Vitalism survived for a while even after the rise of modern atomic theory and the replacement of the Aristotelian elements by those we know today. It first came under question in 1824, when Friedrich Wöhler synthesized oxalic acid, a compound known to occur only in living organisms, from cyanogen. A more decisive experiment was Wöhler's 1828 synthesis of urea from the inorganic saltspotassium cyanate and ammonium sulfate. Urea had long been considered to be an "organic" compound as it was known to occur only in the urine of living organisms. Wöhler's experiments were followed by many others, where increasingly complex "organic" substances were produced from "inorganic" ones without the involvement of any living organism.

Modern classification

Even after vitalism had been disproved, the distinction between "organic" and "inorganic" compounds has been retained through the present. The modern meaning of "organic compound" is any one of them that contains a significant amount of carbon - even though many of the "organic compounds" known today have no connection whatsoever with any substance found in living organisms.

There is no "official" definition of an organic compound. Some text books define an organic compound as one containing one or more C-H bonds; others include C-C bonds in the definition. Others state that if a molecule contains carbon it is organic.

Even the broader definition of "carbon-containing molecules" requires the exclusion of carbon-containing alloys (including steel), a relatively small number of carbon-containing compounds such as metal carbonates and carbonyls, simple oxides of carbon and cyanides, as well as the allotropes of carbon and simple carbon halides and sulfides, which are usually considered to be inorganic.

The "C-H" definition excludes compounds which are historically and practically considered to be organic. Neither urea nor oxalic acid are organic by this definition, yet they were two key compounds in the vitalism debate. The IUPAC Blue Book on organic nomenclature specifically mentions urea and oxalic acid. Other compounds lacking C-H bonds that are also traditionally considered to be organic include benzenehexol, mesoxalic acid, and carbon tetrachloride. Mellitic acid, which contains no C-H bonds, is considered to be a possible organic substance in Martian soil. All do, however, contain C-C bonds.

The "C-H bond only" rule also leads to somewhat arbitrary divisions in sets of carbon-fluorine compounds, as for example Teflon is considered by this rule "inorganic" but Tefzel organic; similarly many Halons are considered inorganic while the rest are organic. For these and other reasons, most sources consider C-H compounds to be only a subset of "organic" compounds.

To summarize: Most carbon-containing compounds are organic, and most compounds with a C-H bond are organic. Not all organic compounds necessarily contain C-H bonds (e.g. urea).


SeeClassification of organic compounds

Organic compounds may be classified in a variety of ways. One major distinction is between natural and synthetic compounds. Organic compounds can also be classified or subdivided by the presence of heteroatoms, e.g. organometallic compounds which feature bonds between carbon and a metal, and From Yahoo Answers

Question:Please explain what parameters we need to consider to decide which is more or less acidic ? And arrange these compounds in order of their decreasing acidic character(most to least) :- 1. Propane [ CH3-CH2-CH3 ] 2. Propene [ CH2=CH2-CH3 ] 3. Propyne [ CH3-C C-H ] 4. Propanol [ CH3-CH2-CH2-OH ] 5. Propanone [ CH3-CO-CH3 ] 6. Propanoic acid [ CH3-CH2-COOH ]

Answers:6 will be the most as it is an acid 4 the OH can under high acidity give up an H not 100% sure on how to rank 1,2,3 (other than close to each other) 3 is slighlty basic so last on it 1 2 3 these are slighlty basic 5 I believe that I put them in order - as I recall double and triple bonds have a Lewis Base aspect to them. "The p electrons are what give alkenes their reactivity. This pair of loosely held electrons can act as a Lewis Base, an electron pair donor. They will react with a Lewis Acid, an electron pair acceptor. " http://butane.chem.uiuc.edu/cyerkes/Chem104ACSpring2009/Lecture_Notes_104/lect16c.html I rec' to the Asker to ask Colin to answer as he does better on Organic than me. I am a little out of expertise here. I hope Colin answers and I, too, shall learn something.

Question:I Have couple of questions which I didnt know how to solve which are about the organic compounds! 1- Organic compunds are also covalent compunds. What properties would you expect organic compund to have as as result? 2- Farmers have been known to taste their soil to detemine whether the soil has the correct acidity for their plants. How would taste help the farmer determine the acidity of the soil? Thnx alot ppl :)

Answers:1) You can expect an organic compound to have lots of Covalent bonds. The elements that have the highest affinity for covalent bonding are O, C, H, N and S, and are therefor most common in all organic molecules. 2) Acidic substances taste sour. Basic substances taste soapy. (bitter and fatty)

Question:How do these organic compounds differentiate? Sugars Organic acids Alcohols Esters Aldahydes I know that for alcohol, it is different because it's carbon chain ends in OH, but how about the rest?

Answers:This is basic organic chemistry. Get very familiar with these terms. Let's take them in a turn to make sense of them...to see how they relate to one another: 1. Aldehydes have a -CH(=O) attached...eg CH3CH(=O), acetaldehyde. (note the O is attached to C via a double bond) 2. If you reduce an aldehyde, you get an alcohol. which has a hydroxyl group attached, eg... CH3CH(=O) + H2 ---> CH3CH2OH, ethanol. 3. If you oxydize an aldehyde, you get an organic (carboxylic) acid, which has a carboxyl group attached, eg CH3CH(=O) + 1/2 O2 ---> CH3COOH, acetic acid. 4. If you react above acid and alcohol, splitting out water, you get an ester, eg CH3COOH + HOCH2CH3 ---> H2O + CH3COOCH2CH3, ethyl acetate. 5. Sugars are polyhydroxyl compounds...also termed carbohydrates because they belong to the group having equal mols of carbon and water in their structures. Glucose is a common one... CH2OHCH2OHCH2OHCH2OHCH2OHCH2OH, and it can be dehydrated to: ...---> 6C + 6H2O Learning in this fashion helps your mind 'pigeon-hole' these concepts so that they are readily available when you need them for more complicated work...which will come soon :-)) Be sure to learn each new concept stepwise as it is presented...don't fall behind.

Question:I'm so confused.. I have little bacground in anything regarding acids and bases in the form of finding their "level." I've found the equation: HA + H20 <=> H30 + A- And I have the following to rank from acidic to basic: Methanol (CH3OH) Methanoic acid (CHOH) Methanamine (CH3NH2) Methanal (CH2O) I've read books, looked at websites, for at least 1.5 hours.. I still don't get it. I have them all drawn out, labled, chemical formulas.. but what do I do now? Thanks. Whoops, I wrote one of them wrong. Methanoic Acid is CH2O2 So how would you find the KB and KA values of the one part. Do you take away the CH's (CH, CH2, CH3, etc) and see what you have left?

Answers:You would do well to know what you expect to do before you start doing it. The main "key" to ranking these compounds is to look for Ka or Kb values in acid or base source material. Substances that have Ka values are acids. The higher the Ka value, the stronger is the acid. The opposite is true for the bases. As for your compounds, methylamine is a weak base (a first cousin to ammonia). As for methanoic acid and methanal, I think you have the same compound twice, what we called formaldehyde in the good old days before the IUPAC police and no prayer in school.

From Youtube

Organic Compounds :Basics of carbohydrates, lipids, proteins, and nucleic acids.

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