DOCSLIB.ORG

4.1.3 Revision Guides Alkenes

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
4.1.3 Revision Guides Alkenes 4.1.3 Alkenes Alkenes contain a carbon- General formula is 2n Alkenes are unsaturated hydrocarbons CnH carbon double bond somewhere in their structure H H H H C C C H C C Ethene Propene H H H H H H H H H Numbers need to be H added to the name when H C C C C H But-2-ene C C C C H But-1-ene positional isomers can occur H H H H H H H H π bonds are exposed and have high C=C double covalent bond electron density. consists of one sigma (σ) bond and one pi (π) bond. They are therefore vulnerable to attack by species which ‘like’ electrons: these C-C sigma bond C-C pi bond species are called electrophiles. Formation of σ bond in alkenes C C two sp2 orbitals (one from each carbon) overlap to sigma σ bond form a single C-C bond called a sigma σ bond Formation of π bond in alkenes p orbitals Rotation can occur around a sigma bond The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule. C-C sigma bond The π bond is weaker than C-C pi bond the σ bond. H H H H The arrangement of bonds around C C C C the >C=C< is planar and has the bond angle 120o H CH3 H H N Goalby chemrevise.org 1 Stereoisomerism Stereoisomers have the same structural formulae but Alkenes can exhibit a type of isomerism have a different spatial arrangement of atoms. called E-Z stereoisomerism E-Z isomers exist due to restricted E-Z stereoisomers arise when: rotation about the C=C bond (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends of the Single carbon-carbon covalent double bond. bonds can easily rotate But-1-ene is a structural isomer of But-2-ene H H two different groups but does not show E-Z isomerism. H H H C C H attached either end of the restricted double bond- H C C H But-1-ene C C leads to EZ isomers C C H H H H H H H Z- but-2-ene H two identical groups attached to one end of the restricted double bond – H no E-Z isomers H These are two isomers as C H the lack of rotation H Skeletal formulae can also represent E-Z isomerism C C around the double bonds H means one cannot be Z-but-2-ene H C switched to the other. H H E-but-2-ene E -but-2-ene Naming E-Z stereoisomers Priority Group: The atom with the bigger First determine the priority groups on both sides of the double bond atomic number is classed as the priority atom Priority Priority group Cl Cl group Cl H side 1 side 2 C C C C Z-1,2-dichloroethene H H H Cl E-1,2-dichloroethene If the priority atom is on the same side of the If the priority atom is on the opposite side of the double bond it is labelled Z from the german double bond it is labelled E from the german zusammen (The Zame Zide!) entgegen (The Epposite side!) Cahn–Ingold–Prelog (CIP) priority rules. 1. Compare the atomic number of the atoms directly attached to each side of the Cl Br priority priority double bond; the atom of higher atomic number is given priority. C C 2. If the atoms are the same, consider the H Cl atoms at distance 2 from the double bond. Make a list of each atom bonded to the one directly attached to the double bond. H C Arrange list in order of decreasing atomic 3 CH3 priority number. Compare the lists atom by atom; at C C the earliest difference, the group containing priority the atom of higher atomic number is given CH H priority 2 H3C N Goalby chemrevise.org 2 cis-trans isomerism is a special case of EIZ isomerism in which two of the substituent groups are the same. H H H H H C C H C H H C C C C H H H H H H C H H Z- but-2-ene E- but-2-ene Can also be called Can also be called Cis- but-2-ene trans- but-2-ene The effect of EZ stereoisomerism on physical properties E-Z stereoisomers can have differing melting and boiling points. δ- δ- Cl δ- Cl Cl H δ+ C C δ+ δ+ C C δ+ H H H Cl δ- Z-1,2-dichloroethene Boiling point =60oC E-1,2-dichloroethene o This molecule is polar. The polar C-Cl bonds are on Boiling point =48 C the same side of the molecule. One side of the This molecule is non- polar. The polar C-Cl molecule is slightly negative. bonds are on opposite sides of the molecule. The intermolecular forces are both London forces The dipoles cancel out. and permanent dipole-dipole attractions. The intermolecular forces are is only London forces so lower boiling point. N Goalby chemrevise.org 3 Addition reactions of alkenes The alkenes are relatively reactive because of the relatively low bond enthalpy of the π-bond. Addition reaction: a reaction where two molecules react 1. Reaction of Alkenes with Hydrogen together to produce one Change in functional group: alkene alkane H H H H Reagent: hydrogen H C C H Conditions: nickel catalyst C C + H2 Type of reaction: Addition/reduction H H H H ethene ethane Electrophilic Addition Reactions of Alkenes The double bonds in alkenes are areas with high Definition Electrophile: an electron pair acceptor electron density. This attracts electrophiles and the alkenes undergo addition reactions 2. Reaction of alkenes with bromine/chlorine H H Change in functional group: alkene dihaloalkane H H Reagent: Bromine H C C H C C + Br Conditions: Room temperature (not in UV light) 2 Mechanism: Electrophilic Addition H H Br Br + Type of reagent: Electrophile, Br 1,2-dibromoethane Type of Bond Fission: Heterolytic As the Br molecule H H 2 H H H H approaches the alkene, the pi bond electrons + The INTERMEDIATE C C H C C H H C C H repel the electron pair in formed, which has a the Br-Br bond. This δ+ positive charge on a H H Br carbon atom is called a INDUCES a DIPOLE. Br2 Br Br Br becomes polar and :Br - CARBOCATION δ+ ELECTROPHILIC (Br ). Brδ- N Goalby chemrevise.org 4 3. Reaction of Hydrogen Bromide with Alkenes Change in functional group: alkenehaloalkane H H H H H H Reagent: HCl or HBr H C C C C H + HBr H C C C C H Conditions: Room temperature Mechanism: Electrophilic Addition H H H H H H Br H Type of reagent: Electrophile, H+ But-2-ene 2-bromobutane HBr is a polar H H H molecule because H H H This reaction can Br is more lead to two H3C C C CH3 + products when the electronegative H3C C C CH3 H3C C C CH3 than H. The H δ + is δ+ alkene is attracted to the H H Br H unsymmetrical - electron-rich pi Brδ :Br - bond. - ‘Markownikoff’s Rule’ H :Br In most cases, bromine will be added to the + H H H H carbon with the fewest hydrogens attached to it C CH2 Major H C C C C H If the alkene is H3C CH3 product unsymmetrical, δ+ δ- :Br - 90% addition of hydrogen H Br H H Br H bromide can lead to H H + two isomeric CH2 CH2 CH2 CH3 Minor H2C CH CH2 CH3 C C CH2 CH3 products. product Br H H 10% WHY? In electrophilic addition to alkenes, the major product is This carbocation intermediate H H H formed via the more stable carbocation intermediate. is more stable because the methyl groups on either side H C C C H In exam answers + of the positive carbon are •Draw out both carbocations and identify as primary, secondary electron releasing and reduce H H and tertiary the charge on the ion which •State which is the more stable carbocation e.g. secondary more stabilises it. stable than primary •State that the more stable carbocation is stabilised because the The order of stability for carbocations is methyl groups on either (or one) side of the positive carbon are tertiary > secondary >primary electron releasing and reduce the charge on the ion. •(If both carbocations are secondary then both will be equally stable and a 50/50 split will be achieved) 4. Reaction of alkenes with steam to form alcohols Industrially alkenes are converted to alcohols in one step. They are This reaction can be called reacted with steam in the presence of an acid catalyst. hydration: a reaction where water is added to a molecule CH2=CH2 (g) + H2O (g) CH3CH2OH (l) Reagent : steam Essential Conditions The high pressures needed mean this cannot be High temperature 300 to 600°C done in the laboratory. It is preferred industrially, however, as there are no waste products and so has High pressure 70 atm a high atom economy. It would also mean separation Catalyst of concentrated H PO of products is easier (and cheaper) to carry out. 3 4 N Goalby chemrevise.org 5 Addition Polymers Addition polymers are formed from alkenes Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds This is called addition polymerisation be able to recognise the repeating unit in a poly(alkene) n Monomer H H H H H H Polymer Ethene polyethene C C C C C C H H H H CH3 H CH3 H CH3 H n C C C C H CH3 H CH3 n propene poly(propene) Add the n’s if writing an equation showing the Poly(propene) is recycled reaction where ‘n’ monomers become ‘n’ repeating units It is best to first draw out H CH3 You should be able e.g.
Load more

Recommended publications
  • Transcription 12.02.15
    Lecture 12A • 02/15/12 We’re going to continue our discussion of conjugation. If we go back and talk about electrons and how they act like waves, [there’s] something known as the particle in a box. You’ve got some kind of box where imagine that the walls are infinitely tall. It’s a one-dimensional system, where the electron, all it can do, is go left and right between the two walls of the box. The electron’s a wave; it can’t exist outside the box, so whatever function, whatever wave we use to describe the electron, has to have a value of zero at one end of the box and zero at the other end of the box. Graphically, what do the solutions look like? We have something like this, where we’re talking about energy potential; it’s infinite at either end of the box, and zero in between. We have an electron that’s bouncing around inside of it. An electron’s a wave, and there’s function that describes that wave. The only way it can fit in this box and physically make sense is if the wave starts and stops and the ends of the box. It turns out that – long, long, long story short – one of the solutions for the Schrödinger equation is just a sine function – actually, it’s part of an exponential version of a sine function. It’s something, at least, we can draw a pretty picture of. That’s why you’ll start with this problem in a discussion of quantum mechanics, because it is solvable, it is only on in dimension, and it’s more humanly possible to discuss.
    [Show full text]
  • Aromaticity Sem- Ii
    AROMATICITY SEM- II In 1931, German chemist and physicist Sir Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties .This is a very popular and useful rule to identify aromaticity in monocyclic conjugated compound. According to which a planar monocyclic conjugated system having ( 4n +2) delocalised (where, n = 0, 1, 2, .....) electrons are known as aromatic compound . For example: Benzene, Naphthalene, Furan, Pyrrole etc. Criteria for Aromaticity 1) The molecule is cyclic (a ring of atoms) 2) The molecule is planar (all atoms in the molecule lie in the same plane) 3) The molecule is fully conjugated (p orbitals at every atom in the ring) 4) The molecule has 4n+2 π electrons (n=0 or any positive integer Why 4n+2π Electrons? According to Hückel's Molecular Orbital Theory, a compound is particularly stable if all of its bonding molecular orbitals are filled with paired electrons. - This is true of aromatic compounds, meaning they are quite stable. - With aromatic compounds, 2 electrons fill the lowest energy molecular orbital, and 4 electrons fill each subsequent energy level (the number of subsequent energy levels is denoted by n), leaving all bonding orbitals filled and no anti-bonding orbitals occupied. This gives a total of 4n+2π electrons. - As for example: Benzene has 6π electrons. Its first 2π electrons fill the lowest energy orbital, and it has 4π electrons remaining. These 4 fill in the orbitals of the succeeding energy level. The criteria for Antiaromaticity are as follows: 1) The molecule must be cyclic and completely conjugated 2) The molecule must be planar.
    [Show full text]
  • 8.3 Bonding Theories >
    8.3 Bonding Theories > Chapter 8 Covalent Bonding 8.1 Molecular Compounds 8.2 The Nature of Covalent Bonding 8.3 Bonding Theories 8.4 Polar Bonds and Molecules 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals Molecular Orbitals How are atomic and molecular orbitals related? 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals • The model you have been using for covalent bonding assumes the orbitals are those of the individual atoms. • There is a quantum mechanical model of bonding, however, that describes the electrons in molecules using orbitals that exist only for groupings of atoms. 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals • When two atoms combine, this model assumes that their atomic orbitals overlap to produce molecular orbitals, or orbitals that apply to the entire molecule. 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals Just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole. • A molecular orbital that can be occupied by two electrons of a covalent bond is called a bonding orbital. 5 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals Sigma Bonds When two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting two atomic nuclei, a sigma bond is formed. • Its symbol is the Greek letter sigma (σ).
    [Show full text]
  • Molecular Orbital Theory
    Molecular Orbital Theory The Lewis Structure approach provides an extremely simple method for determining the electronic structure of many molecules. It is a bit simplistic, however, and does have trouble predicting structures for a few molecules. Nevertheless, it gives a reasonable structure for many molecules and its simplicity to use makes it a very useful tool for chemists. A more general, but slightly more complicated approach is the Molecular Orbital Theory. This theory builds on the electron wave functions of Quantum Mechanics to describe chemical bonding. To understand MO Theory let's first review constructive and destructive interference of standing waves starting with the full constructive and destructive interference that occurs when standing waves overlap completely. When standing waves only partially overlap we get partial constructive and destructive interference. To see how we use these concepts in Molecular Orbital Theory, let's start with H2, the simplest of all molecules. The 1s orbitals of the H-atom are standing waves of the electron wavefunction. In Molecular Orbital Theory we view the bonding of the two H-atoms as partial constructive interference between standing wavefunctions of the 1s orbitals. The energy of the H2 molecule with the two electrons in the bonding orbital is lower by 435 kJ/mole than the combined energy of the two separate H-atoms. On the other hand, the energy of the H2 molecule with two electrons in the antibonding orbital is higher than two separate H-atoms. To show the relative energies we use diagrams like this: In the H2 molecule, the bonding and anti-bonding orbitals are called sigma orbitals (σ).
    [Show full text]
  • Inorganic Chemistry/Chemical Bonding/MO Diagram 1
    Inorganic Chemistry/Chemical Bonding/MO Diagram 1 Inorganic Chemistry/Chemical Bonding/MO Diagram A molecular orbital diagram or MO diagram for short is a qualitative descriptive tool explaining chemical bonding in molecules in terms of molecular orbital theory in general and the Linear combination of atomic orbitals molecular orbital method (LCAO method) in particular [1] [2] [3] . This tool is very well suited for simple diatomic molecules such as dihydrogen, dioxygen and carbon monoxide but becomes more complex when discussing polynuclear molecules such as methane. It explains why some molecules exist and not others, how strong bonds are, and what electronic transitions take place. Dihydrogen MO diagram The smallest molecule, hydrogen gas exists as dihydrogen (H-H) with a single covalent bond between two hydrogen atoms. As each hydrogen atom has a single 1s atomic orbital for its electron, the bond forms by overlap of these two atomic orbitals. In figure 1 the two atomic orbitals are depicted on the left and on the right. The vertical axis always represents the orbital energies. The atomic orbital energy correlates with electronegativity as a more electronegative atom holds an electron more tightly thus lowering its energy. MO treatment is only valid when the atomic orbitals have comparable energy; when they differ greatly the mode of bonding becomes ionic. Each orbital is singly occupied with the up and down arrows representing an electron. The two AO's can overlap in two ways depending on their phase relationship. The phase of an orbital is a direct consequence of the oscillating, wave-like properties of electrons.
    [Show full text]
  • Structure of Benzene, Orbital Picture, Resonance in Benzene, Aromatic Characters, Huckel’S Rule B
    Dr.Mrityunjay Banerjee, IPT, Salipur B PHARM 3rd SEMESTER BP301T PHARMACEUTICAL ORGANIC CHEMISTRY –II UNIT I UNIT I Benzene and its derivatives 10 Hours A. Analytical, synthetic and other evidences in the derivation of structure of benzene, Orbital picture, resonance in benzene, aromatic characters, Huckel’s rule B. Reactions of benzene - nitration, sulphonation, halogenationreactivity, Friedelcrafts alkylation- reactivity, limitations, Friedelcrafts acylation. C. Substituents, effect of substituents on reactivity and orientation of mono substituted benzene compounds towards electrophilic substitution reaction D. Structure and uses of DDT, Saccharin, BHC and Chloramine Benzene and its Derivatives Chemists have found it useful to divide all organic compounds into two broad classes: aliphatic compounds and aromatic compounds. The original meanings of the words "aliphatic" (fatty) and "aromatic" (fragrant/ pleasant smell). Aromatic compounds are benzene and compounds that resemble benzene in chemical behavior. Aromatic properties are those properties of benzene that distinguish it from aliphatic hydrocarbons. Benzene: A liquid that smells like gasoline Boils at 80°C & Freezes at 5.5°C It was formerly used to decaffeinate coffee and component of many consumer products, such as paint strippers, rubber cements, and home dry-cleaning spot removers. A precursor in the production of plastics (such as Styrofoam and nylon), drugs, detergents, synthetic rubber, pesticides, and dyes. It is used as a solvent in cleaning and maintaining printing equipment and for adhesives such as those used to attach soles to shoes. Benzene is a natural constituent of petroleum products, but because it is a known carcinogen, its use as an additive in gasoline is now limited. In 1970s it was associated with leukemia deaths.
    [Show full text]
  • Alkenes and Alkynes
    02/21/2019 CHAPTER FOUR Alkenes and Alkynes H N O I Cl C O C O Cl F3C C Cl C Cl Efavirenz Haloprogin (antiviral, AIDS therapeutic) (antifungal, antiseptic) Chapter 4 Table of Content * Unsaturated Hydrocarbons * Introduction and hybridization * Alkenes and Alkynes * Benzene and Phenyl groups * Structure of Alkenes, cis‐trans Isomerism * Nomenclature of Alkenes and Alkynes * Configuration cis/trans, and cis/trans Isomerism * Configuration E/Z * Physical Properties of Hydrocarbons * Acid‐Base Reactions of Hydrocarbons * pka and Hybridizations 1 02/21/2019 Unsaturated Hydrocarbons • Unsaturated Hydrocarbon: A hydrocarbon that contains one or more carbon‐carbon double or triple bonds or benzene‐like rings. – Alkene: contains a carbon‐carbon double bond and has the general formula CnH2n. – Alkyne: contains a carbon‐carbon triple bond and has the general formula CnH2n‐2. Introduction Alkenes ● Hydrocarbons containing C=C ● Old name: olefins • Steroids • Hormones • Biochemical regulators 2 02/21/2019 • Alkynes – Hydrocarbons containing C≡C – Common name: acetylenes Unsaturated Hydrocarbons • Arene: benzene and its derivatives (Ch 9) 3 02/21/2019 Benzene and Phenyl Groups • We do not study benzene and its derivatives until Chapter 9. – However, we show structural formulas of compounds containing a phenyl group before that time. – The phenyl group is not reactive under any of the conditions we describe in chapters 5‐8. Structure of Alkenes • The two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°. 4 02/21/2019 Structure of Alkenes • Figure 4.1 According to the orbital overlap model, a double bond consists of one bond formed by overlap of sp2 hybrid orbitals and one bond formed by overlap of parallel 2p orbitals.
    [Show full text]
  • Reactions of Aromatic Compounds Just Like an Alkene, Benzene Has Clouds of  Electrons Above and Below Its Sigma Bond Framework
    Reactions of Aromatic Compounds Just like an alkene, benzene has clouds of electrons above and below its sigma bond framework. Although the electrons are in a stable aromatic system, they are still available for reaction with strong electrophiles. This generates a carbocation which is resonance stabilized (but not aromatic). This cation is called a sigma complex because the electrophile is joined to the benzene ring through a new sigma bond. The sigma complex (also called an arenium ion) is not aromatic since it contains an sp3 carbon (which disrupts the required loop of p orbitals). Ch17 Reactions of Aromatic Compounds (landscape).docx Page1 The loss of aromaticity required to form the sigma complex explains the highly endothermic nature of the first step. (That is why we require strong electrophiles for reaction). The sigma complex wishes to regain its aromaticity, and it may do so by either a reversal of the first step (i.e. regenerate the starting material) or by loss of the proton on the sp3 carbon (leading to a substitution product). When a reaction proceeds this way, it is electrophilic aromatic substitution. There are a wide variety of electrophiles that can be introduced into a benzene ring in this way, and so electrophilic aromatic substitution is a very important method for the synthesis of substituted aromatic compounds. Ch17 Reactions of Aromatic Compounds (landscape).docx Page2 Bromination of Benzene Bromination follows the same general mechanism for the electrophilic aromatic substitution (EAS). Bromine itself is not electrophilic enough to react with benzene. But the addition of a strong Lewis acid (electron pair acceptor), such as FeBr3, catalyses the reaction, and leads to the substitution product.
    [Show full text]
  • 18. Nucleophilic Sigma Bonds
    Professor David L. Van Vranken Chemistry 201: Organic Reaction Mechanisms I Topic 18: Nucleophilic Sigma Bonds E+ E+ E+ R Li R C R H References: Literature cited Recall the Six Types of Canonical Frontier Orbitals ■ We’ve already discussed the interactions of 3 types of filled orbitals and 3 types of unfilled orbitals. If all things are equal then C-C and C-H sigma bonds should be the least reactive type of nucleophiles but if we replace C with Li to make a C-Li bond then the bonds became exquisitely reactive. σ* π* Li E+ p H E+ - E n B MO H H π H H E+ σ -O ■ We need to discuss the nucleophilicity of sigma bonds more broadly. Periodic Trends - Electronegativity ■ Here are some periodic trends you should know… electronegative rapid atom transfer pi character weaker, more nucleophilic bonds electropositive http://www.green-planet-solar-energy.com/electronegativity-values.html Frontier Molecular Orbital Energies Predict Reactivity ■ What happens when you replace carbon with an electropositive atom? Bonds to electropositive atoms can be highly nucleophilic. E+ faster .. nO(Li) O Li faster nO(Be) E E+ E MO MO nO(B) E+ slower σC-Li H3C Li .. nO(C) O CH3 σC-Be σC-B E+ E+ σC-C H3C CH3 σO-Li O Li σC-N ■ For bonds to O and N, the non-bonding FMO (i.e., the lone pair) is still more reactive than the sigma bond FMO. Alkyllithiums: Frontside vs. Backside Attack ■ Frontside attack is most common for R-Li. Et Et Still, W.
    [Show full text]
  • II. Nomenclature Rules for Alkenes 1. the Parent Name Will Be the Longest
    1 Lecture 9 II. Nomenclature Rules For Alkenes 1. The parent name will be the longest carbon chain that contains both carbons of the double bond. Drop the -ane suffix of the alkane name and add the –ene suffix. Never name the double bond as a prefix. If a double bond is present, you have an alkene, not an alkane. alkane + -ene = alkene 2. Begin numbering the chain at the end nearest the double bond. Always number through the double bond and identify its position in the longest chain with the lower number. In the older IUPAC rules the number for the double bond was placed in front of the stem name with a hyphen. Under the newer rules, the number for the double bond is placed right in front of “ene”, with hyphens. We will use the newer rules for specifying the location of pi bonds. 1 2 3456 H3CCHCH CH 2 CH2 CH3 hex-2-ene (newer rules) 2-hexene (older rules) 3. Indicate the position of any substituent group by the number of the carbon atom in the parent (longest) chain to which it is attached. CH 1 2 345 3 H3CCHCHCHCH2 CH CH3 6 7 CH3 Numbering is determined by the double bond, not the branches, because the double bond has 5,6-dimethylhept-3-ene (newer rules) higher priority than any alkyl branch. 5,6-dimethyl-3-heptene (older rules) 4. Number cycloalkenes so that the double bond is 1,2 (number through the double bond). Number in the direction about the ring so that the lowest number is used at the first point of difference.
    [Show full text]
  • I 1.9 Aromotic Compounds
    5t8 CHAPTERI I CarbonChains and Rings PRACTICE EXERCISEII.S \ Draw structural formulas for the following alkenes.If a compound has geometric isomers, draw both the cls and the transforms. (a) l-pentene (b) 2-hexene (c) 2-methyl-2-hexene (d) 2,3-dimethyl-2-butene Alkynes Organic compounds containing carbon-carbon triple bonds are called alkynes. Like alkenes,alkynes are unsaturated compounds. This is the car- bon-carbon triple bond found in alkynes: -C:C- Allqmes are quite rare in nature. The simplest alkyne is the gasethlme, C2H2. The common name for ethyne is acetylene-the fuel burned in oxyacety- lene torches used in welding. The single bonds that extend from the car- bons involved in the carbon-carbon Figurel l .l0 triple bond of ethpre are separatedby (Fig. Ball-and-stickmodel of ethyne the maximum bond angle, IB0 degrees f f .10). Ethyne is a linear (commonname acetylene). molecule. The development of an orbital picture for ethyne requires us to go through the same steps that we used for methane and ethene: hybridiza- tion of carbon atomic orbitals and orbital overlap. As with methane and ethene,remember that each orbital representsone electron.The descrip- tion of ethyne that best flts the experimental data is obtained if the 2s atomic orbital of carbon is mixed with only one of the three available 2p atomic orbitals (Fig. f 1.11).The result of this mixing is two sp atomic orbitals; two 2p orbitals remain unused. Overlap of one of the sp hybrid orbitals of one carbon with that of another carbon producesa carbon-car- bon sigma bond.
    [Show full text]
  • Valence Bond Theory
    Valence Bond Theory • A bond is a result of overlapping atomic orbitals from two atoms. The overlap holds a pair of electrons. • Normally each atomic orbital is bringing one electron to this bond. But in a “coordinate covalent bond”, both electrons come from the same atom • In this model, we are not creating a new orbital by the overlap. We are simply referring to the overlap between atomic orbitals (which may or may not be hybrid) from two atoms as a “bond”. Valence Bond Theory Sigma (σ) Bond • Skeletal bonds are called “sigma” bonds. • Sigma bonds are formed by orbitals approaching and overlapping each other head-on . Two hybrid orbitals, or a hybrid orbital and an s-orbital, or two s- orbitals • The resulting bond is like an elongated egg, and has cylindrical symmetry. Acts like an axle • That means the bond shows no resistance to rotation around a line that lies along its length. Pi (π) Bond Valence Bond Theory • The “leftover” p-orbitals that are not used in forming hybrid orbitals are used in making the “extra” bonds we saw in Lewis structures. The 2 nd bond in a double bond nd rd The 2 and 3 bonds in a triple bond /~harding/IGOC/P/pi_bond.html • Those extra bonds form only after the atoms are brought together by the formation of the skeletal bonds made by www.chem.ucla.edu hybrid orbitals. • The “extra” π bonds are always associated with a skeletal bond around which they form. • They don’t form without a skeletal bond to bring the p- orbitals together and “support” them.
    [Show full text]