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Haloalkane

The haloalkanes (also known as halogenoalkanes or alkyl halides) are a group of chemical compounds derived from alkanes containing one or more halogens. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially and, consequently, are known under many chemical and commercial names. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes which contain chlorine, bromine, and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases. Methyl iodide, a naturally occurring substance, however, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, see Halon.

Haloalkanes have been known for centuries. Ethyl chloride was produced synthetically in the 15th century. The systematic synthesis of such compounds developed in the 19th century in step with the development of organic chemistry and the understanding of the structure of alkanes. Methods were developed for the selective formation of C-halogen bonds. Especially versatile methods included the addition of halogens to alkenes, hydrohalogenation of alkenes, and the conversion of alcohols to alkyl halides. These methods are so reliable and so easily implemented that haloalkanes became cheaply available for use in industrial chemistry because the halide could be further replaced by other functional groups.

While most haloalkanes are human-produced, non-artificial-source haloalkanes do occur on Earth, mostly through enzyme-mediated synthesis by bacteria, fungi, and especially sea macroalgae (seaweeds). More than 1600 halogenated organics have been identified, with bromoalkanes being the most common haloalkanes. Brominated organics in biology range from biologically-produced methyl bromide to non-alkane aromatics and unsaturates (indoles, terpenes, acetogenins, and phenols). Halogenated alkanes in land plants are more rare, but do occur, as for example the fluoroacetate produced as a toxin by at least 40 species of known plants. Specific dehalogenase enzymes in bacteria which remove halogens from haloalkanes, are also known.

Classes of haloalkanes

From the structural perspective, haloalkanes can be classified according to the connectivity of the carbon atom to which the halogen is attached. In primary (1°) haloalkanes, the carbon that carries the halogen atom is only attached to one other alkyl group. An example is chloroethane (CH3CH2Cl). In secondary (2°) haloalkanes, the carbon that carries the halogen atom has two C-C bonds. In tertiary (3°) haloalkanes, the carbon that carries the halogen atom has three C-C bonds.

Haloalkanes can also be classified according to the type of halogen. Haloalkanes containing carbon bonded to fluorine, chlorine, bromine, and iodine results in organofluorine, organochlorine, organobromine and organoiodine compounds, respectively. Compounds containing more than one kind of halogen are also possible. Several classes of widely used haloalkanes are classified in this way Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). These abbreviations are particularly common in discussions of the environmental impact of haloalkanes.

Properties

Haloalkanes generally resemble the parent alkanes in being colorless, relatively odorless, and hydrophobic. Their boiling points are higher than the corresponding alkanes and scale with the atomic weight and number of halides. This is due to the increased strength of the intermolecular forces—from London dispersion to dipole-dipole interaction because of the increased polarity. Thus carbon tetraiodide (CI4) is a solid whereas carbon tetrafluoride (CF4) is a gas. As they contain fewer C-H bonds, halocarbons are less flammable than alkanes, and some are used in fire extinguishers. Haloalkanes are better solvents than the corresponding alkanes because of their increased polarity. Haloalkanes containing halogens other than fluorine are more reactive than the parent alkanes - it is this reactivity that is the basis of most controversies. Many are alkylating agents, with primary haloalkanes and those containing heavier halogens being the most active (fluoroalkanes do not act as alkylating agents under normal conditions). The ozone-depleting abilities of the CFC's arises from the photolability of the C-Cl bond.

Occurrence

Haloalkanes are of wide interest because they are widespread and have diverse beneficial and detrimental impacts. The oceans are estimated to release 1-2 million tons of bromomethane annually.

A large number of pharmaceuticals contain halogens, especially fl

Essential nutrient

An essential nutrient is a nutrient required for normal body functioning that either cannot be synthesized by the body at all, or cannot be synthesized in amounts adequate for good health (e.g. niacin, choline), and thus must be obtained from a dietary source. Essential nutrients are also defined by the collective physiological evidence for their importance in the diet, as represented in e.g. US government approved tables for Dietary Reference Intake.

Some categories of essential nutrients include vitamins, dietary minerals, essential fatty acids, and essential amino acids. Water and oxygen are also essential for human health and life, as oxygen cannot be synthesized by the body, and water, while a biochemical reaction product of metabolism, is not created in sufficient amounts. Both are necessary as biochemical reactants in some processes, and water is used in various ways such as a solvent, carrier, coolant, and integral polar structural member, but both are often not included as nutrients.

Different species have very different essential nutrients. For example, most mammals synthesize their own ascorbic acid, and it is therefore not considered an essential nutrient for such species. It is, however, an essential nutrient for human beings, who require external sources of ascorbic acid (known as Vitamin C in the context of nutrition).

Many essential nutrients are toxic in large doses (see hypervitaminosis or the nutrient pages themselves below). Some can be taken in amounts larger than required in a typical diet, with no apparent ill effects. Linus Pauling said of vitamin B3, (either niacin or niacinamide), "What astonished me was the very low toxicity of a substance that has such very great physiological power. A little pinch, 5 mg, every day, is enough to keep a person from dying of pellagra, but it is so lacking in toxicity that ten thousand times as much can [sometimes] be taken without harm."

Fatty acids

Amino acids

Vitamins

Dietary minerals

  • Calcium (Ca)
  • Chloride (Cl−)
  • Chromium (Cr)
  • Cobalt (Co) (as part of Vitamin B-12)
  • Copper (Cu)
  • Iodine (I)
  • Iron (Fe)
  • Magnesium (Mg)
  • Manganese (Mn)
  • chemistry, an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids (see acid-base reaction theories). Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.

    The electrophiles attack the most electron-populated part of a nucleophile. The electrophiles frequently seen in the organic syntheses are cations such as H+ and NO+, polarized neutral molecules such as HCl, alkyl halides, acyl halides, and carbonyl compounds, polarizable neutral molecules such as Cl2 and Br2, oxidizing agents such as organic peracids, chemical species that do not satisfy the octet rule such as carbenes and radicals, and some lewis acids such as BH3 and DIBAL.

    Electrophiles in organic chemistry

    Alkenes

    Electrophilic addition is one of the three main forms of reaction concerning alkenes. They consist of:

    Addition of halogens

    These occur between alkenes and electrophiles, often halogens as in halogen addition reactions. Common reactions include use of bromine water to titrate against a sample to deduce the number of double bonds present. For example, ethene + bromine→ 1,2-dibromoethane:

    C2H4 + Br2→ BrCH2CH2Br

    This takes the form of 3 main steps shown below;

    1. Forming of a π-complex
    2. :The electrophilic Br-Br molecule interacts with electron-rich alkene molecule to form a π-complex1.
    3. Forming of a three-membered bromonium ion
    4. :The alkene is working as an electron donor and bromine as an electrophile. The three-membered bromonium ion2 consisted with two carbon atoms and a bromine atom forms with a release of Br−.
    5. Attacking of bromide ion
    6. :The bromonium ion is opened by the attack of Br− from the back side. This yields the vicinal dibromide with an antiperiplanar configuration. When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether.

    This process is called AdE2 mechanism.Iodine (I2), chlorine (Cl2), sulfenyl ion (RS+), mercury cation (Hg2+), and dichlorocarbene (:CCl2) also react through similar pathways. The direct conversion of 1 to 3 will appear when the Br− is large excess in the reaction medium. A β-bromo carbenium ion intermediate may be predominant instead of 3 if the alkene has a cation-stabilizing substituent like phenyl group. There is an example of the isolation of the bromonium ion 2.

    Addition of hydrogen halides

    Hydrogen halides such as hydrogen chloride (HCl) adds to alkenes to give alkyl halide in hydrohalogenation. For example, the reaction of HCl with ethylene furnishes chloroethane. The reaction proceeds with a cation intermediate, being different from the above halogen addition. An example is shown below:

    1. Proton (H+) adds (by working as an electrophile) to one of the carbon atoms on the alkene to form cation 1.
    2. Chloride ion (Cl−) combines with the cation 1 to form the adducts 2 and 3.

    In this manner, the stereoselectivity of the product, that is, from which side Cl− will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H+ is usually decided by Markovnikov's rule. Thus, H+ attacks the carbon atom that carries fewer substituents so as the more stabilized carbo


From Yahoo Answers

Question:

Answers:Elemental iodine is only slightly soluble in water because of its lack of polarity. Adding potassium iodide to form negatively charged iodide can increase its solubility because of the hydrogen bonds present in water, which can dissolve ions. However, uncharged elemental iodine is more soluble in acetone, an organic compound, because of its lack of polarity.

Question:Can an tertiary haloalkane undergo nucelophilic substitution with Iodine?

Answers:Tertiary substrates usually follow the SN1 mechanism. This follows the carbocation mechanism. SN1 is usaully carried out in a polar solvent, which aids the formation of nucleophiles. Any compound of iodine will have a lattice energy lesser than its corresponding bromide and chloride. So, we can compensate the energy loss in bond breaking form the making up of the crystal lattice. Iodide radical is also more nucleophilic than the other halide radicals. So we tertiary haloalkanes will react with iodides like NaI to form the corresponding iodides and sodium halides. This is called the Coanat-Finkelstein Reaction. {Tertiary haloalkanes will not react with the SN2 reaction meachanism, so the question of steric hinderence does not arise}

Question:Select all substances with an acceptor atom that can hydrogen bond to a hydrogen atom that is part of a polar covalent bond. O2 dioxygen CH3CO2H acetic acid CBr4 carbon tetrabromide IF7 iodine heptafluoride HCl hydrogen chloride CH3CH2OH ethanol

Answers:CH3COOH and C2H5OH.

Question:Can anyone tell me which of these substances are polar and which are nonpolar??? SOLVENTS: water ethanol (C2H5OH) acetone ((CH3)2CO) hexane (C6H14) vegetable oil (CH3(CH2)7CH= CH(CH2)7COOH) isopropyl alcohol (C3H7OH) SOLUTES citric acid (CH2(COOH)- COH(COOH)- CH2(COOH)) gelatin (protein) glycerine (C3H5(OH)3) glycine (NH2CH2COOH) iodine (I2) oleic acid (CH3(CH2)7CH= CH(CH2)7COOH) sodium chloride sucrose (C12H22O11) Vaseline petroleum jelly (hydrocarbon mixture) sodium bicarbonate (NaHCO3) corn starch

Answers:Try to get a periodic table which shows electronegativity numbers. For a given bond between 2 elements, subtract the electronegativity numbers from each other. If the difference is great than 0.5, the bond will likely be polar. Without going through your whole list, items such as water, ethanol and sodium chloride are clearly polar. acetone, hexane, and oils are non-polar.

From Youtube

Performing the Iodine Clock Reaction :This experiment demonstrates the iodine clock reaction between iodide and persulfate ions, using thiosulfate as the 'clock'. After some introduction details, three experiments are performed: studying the effect of concentration to determine the orders of reactants (3:01), studying the effect of temperature to determine the activation energy (7:47) and studying the effect of solvent polarity (9:42).