However, in the trigonal-pyramidal configuration one hydrogen the apex is structurally different from the other three the pyramid base. Substitution in this case should give two different CH 3 Cl compounds if all the hydrogens react. In the case of disubstitution, the tetrahedral configuration of methane would lead to a single CH 2 Cl 2 product, but the other configurations would give two different CH 2 Cl 2 compounds.
These substitution possibilities are shown in the above insert. Structural Formulas It is necessary to draw structural formulas for organic compounds because in most cases a molecular formula does not uniquely represent a single compound. Different compounds having the same molecular formula are called isomers , and the prevalence of organic isomers reflects the extraordinary versatility of carbon in forming strong bonds to itself and to other elements.
When the group of atoms that make up the molecules of different isomers are bonded together in fundamentally different ways, we refer to such compounds as constitutional isomers. There are seven constitutional isomers of C 4 H 10 O, and structural formulas for these are drawn in the following table. These formulas represent all known and possible C 4 H 10 O compounds, and display a common structural feature.
There are no double or triple bonds and no rings in any of these structures. Note that each of the carbon atoms is bonded to four other atoms, and is saturated with bonding partners. Simplification of structural formulas may be achieved without any loss of the information they convey. In condensed structural formulas the bonds to each carbon are omitted, but each distinct structural unit group is written with subscript numbers designating multiple substituents, including the hydrogens.
Shorthand line formulas omit the symbols for carbon and hydrogen entirely. Each straight line segment represents a bond, the ends and intersections of the lines are carbon atoms, and the correct number of hydrogens is calculated from the tetravalency of carbon.
Non-bonding valence shell electrons are omitted in these formulas. Developing the ability to visualize a three-dimensional structure from two-dimensional formulas requires practice, and in most cases the aid of molecular models.
As noted earlier, many kinds of model kits are available to students and professional chemists, and the beginning student is encouraged to obtain one.
Constitutional isomers have the same molecular formula, but their physical and chemical properties may be very different. For an example Click Here. Distinguishing Carbon Atoms When discussing structural formulas, it is often useful to distinguish different groups of carbon atoms by their structural characteristics. The three C 5 H 12 isomers shown below illustrate these terms.
Structural differences may occur within these four groups, depending on the molecular constitution. A consideration of molecular symmetry helps to distinguish structurally equivalent from nonequivalent atoms and groups. The ability to distinguish structural differences of this kind is an essential part of mastering organic chemistry. It will come with practice and experience. Our ability to draw structural formulas for molecules is remarkable. To see how this is done Click Here. Formula Analysis.
Although structural formulas are essential to the unique description of organic compounds, it is interesting and instructive to evaluate the information that may be obtained from a molecular formula alone. Three useful rules may be listed: The number of hydrogen atoms that can be bonded to a given number of carbon atoms is limited by the valence of carbon. The origin of this formula is evident by considering a hydrocarbon made up of a chain of carbon atoms.
Here the middle carbons will each have two hydrogens and the two end carbons have three hydrogens each. Thus, when even-valenced atoms such as carbon and oxygen are bonded together in any number and in any manner, the number of remaining unoccupied bonding sites must be even.
If these sites are occupied by univalent atoms such as H, F, Cl, etc. If the four carbon atoms form a ring, two hydrogens must be lost. Similarly, the introduction of a double bond entails the loss of two hydrogens, and a triple bond the loss of four hydrogens.
By rule 2 m must be an even number, so if m The presence of one or more nitrogen atoms or halogen substituents requires a modified analysis. The above formula may be extended to such compounds by a few simple principles: The presence of oxygen does not alter the relationship. All halogens present in the molecular formula must be replaced by hydrogen.
Each nitrogen in the formula must be replaced by a CH moiety. However, the structures of some compounds and ions cannot be represented by a single formula. For clarity the two ambiguous bonds to oxygen are given different colors in these formulas. If only one formula for sulfur dioxide was correct and accurate, then the double bond to oxygen would be shorter and stronger than the single bond. This averaging of electron distribution over two or more hypothetical contributing structures canonical forms to produce a hybrid electronic structure is called resonance.
Likewise, the structure of nitric acid is best described as a resonance hybrid of two structures, the double headed arrow being the unique symbol for resonance. The above examples represent one extreme in the application of resonance. Here, two structurally and energetically equivalent electronic structures for a stable compound can be written, but no single structure provides an accurate or even an adequate representation of the true molecule.
In cases such as these, the electron delocalization described by resonance enhances the stability of the molecules, and compounds or ions composed of such molecules often show exceptional stability. The electronic structures of most covalent compounds do not suffer the inadequacy noted above. Nevertheless, the principles of resonance are very useful in rationalizing the chemical behavior of many such compounds. For example, the carbonyl group of formaldehyde the carbon-oxygen double bond reacts readily to give addition products.
The course of these reactions can be explained by a small contribution of a dipolar resonance contributor, as shown in equation 3. Here, the first contributor on the left is clearly the best representation of this molecular unit, since there is no charge separation and both the carbon and oxygen atoms have achieved valence shell neon-like configurations by covalent electron sharing. If the double bond is broken heterolytically, formal charge pairs result, as shown in the other two structures.
The preferred charge distribution will have the positive charge on the less electronegative atom carbon and the negative charge on the more electronegative atom oxygen. Therefore the middle formula represents a more reasonable and stable structure than the one on the right.
The application of resonance to this case requires a weighted averaging of these canonical structures. The double bonded structure is regarded as the major contributor, the middle structure a minor contributor and the right hand structure a non-contributor. Since the middle, charge-separated contributor has an electron deficient carbon atom, this explains the tendency of electron donors nucleophiles to bond at this site. The basic principles of the resonance method may now be summarized.
These are the canonical forms to be considered, and all must have the same number of paired and unpaired electrons.
The following factors are important in evaluating the contribution each of these canonical structures makes to the actual molecule. The stability of a resonance hybrid is always greater than the stability of any canonical contributor.
Consequently, if one canonical form has a much greater stability than all others, the hybrid will closely resemble it electronically and energetically. This is the case for the carbonyl group eq.
On the other hand, if two or more canonical forms have identical low energy structures, the resonance hybrid will have exceptional stabilization and unique properties. This is the case for sulfur dioxide eq. To illustrate these principles we shall consider carbon monoxide eq. In each case the most stable canonical form is on the left.
For carbon monoxide, the additional bonding is more important than charge separation. Furthermore, the double bonded structure has an electron deficient carbon atom valence shell sextet. A similar destabilizing factor is present in the two azide canonical forms on the top row of the bracket three bonds vs. The bottom row pair of structures have four bonds, but are destabilized by the high charge density on a single nitrogen atom.
All the examples on this page demonstrate an important restriction that must be remembered when using resonance. No atoms change their positions within the common structural framework. Only electrons are moved. A more detailed model of covalent bonding requires a consideration of valence shell atomic orbitals. The spatial distribution of electrons occupying each of these orbitals is shown in the diagram below.
Very nice displays of orbitals may be found at the following sites: J. Gutow, Univ. Wisconsin Oshkosh R. Spinney, Ohio State M. Winter, Sheffield University. By now the fire had grown too fierce to combat with the crude firefighting methods of the day, which consisted of little more than bucket brigades armed with wooden pails of water.
The usual solution during a fire of such size was to demolish every building in the path of the flames in order to deprive the fire of fuel, but the city's mayor hesitated, fearing the high cost of rebuilding. Meanwhile, the fire spread out of control, doing far more damage than anyone could possibly have managed. Extract taken from the diary of Samuel Pepys.
The writer:. Structure The structure of a text is how it is organised and how its parts fit together. Structural features Feature Purpose Effect on the reader openings The start of a text must interest the reader.
Comment on how the writer introduces ideas and raises questions. Analyse what is implied, eg a gloomy landscape implies an unhappy situation - what is causing that unhappiness? What will happen next? Comment on how this change is effective, eg creates contrast. Comment on the effect a drastic difference produces. Repetitive features can highlight key meanings, indicate a development or show a lack of change. Ask what effect is created by altering the pace, eg a slow pace builds tension or suggests boredom, a quicker pace may suit a piece about things happening at speed.
Comment on how time is used to speed up or slow down the pace of the text. Comment on how the order of events introduces and prioritises key ideas — and how this engages the reader. Think about how the reader feels at the end.
Have their feelings changed since the opening? How does dialogue move the text forward? How do they guide readers through a text? Why does the writer summarise certain points? Comment on how sentence structures affect the fluency of the text, eg a sudden short sentence could reveal shocking information. Comment on how paragraph lengths affect the development of the text, eg a final paragraph might summarise key points in an argument. Structure of a non-fiction text The structure of a non-fiction piece could be: chronological — in date or time order prioritised — the most important facts first like a news article separated into blocks by subheadings — eg in a feature article question and answer — eg in information leaflets problem and solution — eg in agony aunt columns, or self-help guides letter structure — a salutation Dear… and an appropriate ending Yours sincerely… starting in the middle of an event, then providing further information to give several possible viewpoints Using paragraphs to structure a text Look at the way the key ideas in a piece are ordered.
September 2, It was a small mistake, but with great consequences. The start of a text must interest the reader. This is what the writer focuses on as the text develops. Changes in ideas and perspectives, eg outside to inside. The differences between two things. When words, phrases or ideas are repeated for effect.
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