Carbon and its Compounds

1. Why Carbon Must Share

The food you eat, the clothes you wear, the cosmetics you use and the fuels that run automobiles are all compounds of carbon. Carbon was discovered in prehistory and known to the ancients, who made charcoal by burning organic material.

Carbon is a non-metal in group 14 (IV A), whose elements have four valence electrons. Its ground-state configuration is

₆C: 1s² 2s² 2p²

To complete an octet it would have to gain four electrons and become C⁴⁻. The chapter rules this out with a specific argument: carbon's electronegativity is only 2.5 and its nucleus has only six protons, so it would be difficult for six protons to hold ten electrons. C⁴⁻ does not form easily.

Could it instead lose four electrons and reach the helium configuration? Forming C⁴⁺ requires a huge amount of energy, not normally available, so that too is a remote possibility.

Carbon therefore satisfies its tetravalency by sharing — it forms four covalent bonds, with its own atoms or with atoms of other elements.

The ways it can do that:

The chapter is allotted 15 periods across December and January — the largest allotment in the book, shared with no other chapter — and runs from textbook page 253 to page 292.

2. Promotion and Hybridisation

Why four unpaired electrons

The ground state 1s² 2s² 2pₓ¹ 2p_y¹ 2p_z⁰ has only two unpaired electrons. To bond four times, carbon is described in an excited state:

1s² 2s¹ 2pₓ¹ 2p_y¹ 2p_z¹

Where does the energy for that promotion come from? The book answers it carefully: a free carbon atom is not in the excited state under normal conditions. When carbon is ready to bond, the energy for excitation is taken from bond energies — the energy liberated when the bonds themselves form. Forming four bonds rather than two releases more energy and leaves the molecule more stable, and the 2s to 2p energy gap is very small.

Why the four bonds are identical

Promotion alone is not enough. It leaves four unpaired electrons, but in two different kinds of orbital with different energies — and that cannot give four identical bonds. In methane all four C-H bonds are identical and the H-C-H angle is 109° 28′.

Hybridisation — introduced by Linus Pauling in 1931 — is the redistribution of orbitals of almost equal energy in an atom to give an equal number of new orbitals with identical energy and shape. The new orbitals are hybrid orbitals.

The book's Do you know? note on Pauling is worth keeping: he was acknowledged as the most influential chemist, and is the only person ever to receive two unshared Nobel Prizes — Chemistry in 1954 and Peace in 1962.

The three cases

sp³, in methane. The 2s and all three 2p orbitals intermix into four identical sp³ orbitals, each taking one electron by Hund's rule. Four orbitals in an outer shell orient towards the four corners of a tetrahedron to minimise repulsion, with the nucleus at the centre — hence 109° 28′. Four hydrogen s orbitals overlap them to give four sp³-s sigma bonds of equal energy.

sp², in ethene. Each carbon mixes one s and two p orbitals into three sp² orbitals at 120°, leaving one p orbital (p_z) unhybridised. One sp² of each carbon overlap to give an sp²-sp² sigma bond; the other two on each carbon take hydrogens; and the two unhybridised p_z orbitals overlap laterally to form a π bond. So C=C is one σ plus one π.

sp, in ethyne. Each carbon mixes one s and one p orbital into two sp orbitals, leaving two unhybridised p orbitals. One sp-sp sigma bond joins the carbons, the other sp orbitals take hydrogens, and the two pairs of unhybridised p orbitals overlap laterally to give two π bonds. So ethyne has three σ bonds and two π bonds in all.

The Think and discuss box asks you to order the bond distances and bond energies across ethyne, ethene and ethane, and to give the H-C-H angles in CH₄, C₂H₄ and C₂H₂.

3. Allotropes of Carbon

Allotropy is the property of an element to exist in two or more physical forms with more or less similar chemical properties but different physical properties. The forms are allotropes, and they arise from differences in the arrangement of atoms.

Carbon's allotropes are of two kinds:

• Amorphous forms — coal, coke, wood charcoal, animal charcoal, lamp black, gas carbon, petroleum coke, sugar charcoal.
• Crystalline forms — diamond, graphite and buckminsterfullerene.

Diamond and graphite form covalent network structures; buckminsterfullerene is a molecular solid of discrete C₆₀ molecules.

Diamond

Each carbon is sp³ hybridised, so each has a tetrahedral environment in a three-dimensional structure. Because the C-C bonds are very strong, any attempt to distort the structure requires a large amount of energy — which is why diamond is one of the hardest materials known.

Graphite

Graphite forms a two-dimensional layer structure with C-C bonds within the layers and relatively weak interactions between them. Within a layer each carbon is in a trigonal planar environment, consistent with sp² hybridisation, and each carbon keeps one unhybridised p orbital. Those p orbitals interact to form a π system delocalised over the whole layer.

The layers are separated by 3.35 Å and held by London dispersion forces, which are weakened by the presence of water molecules — so graphite cleaves easily. That explains both its uses: lubricant, and the "lead" in pencils.

When you write with a pencil, the interlayer attractions break down and leave graphite layers on the paper. Pencil marks rub out easily because those layers do not bind strongly to the paper. And graphite conducts electricity because of the delocalised π electron system.

Buckminsterfullerene

Fullerenes are molecules composed entirely of carbon, arranged as a hollow sphere, ellipsoid or tube. They form when vaporised carbon condenses in an atmosphere of an inert gas. Spherical ones are called buckyballs.

C₆₀ has the shape of a soccer ball, with 12 pentagonal and 20 hexagonal faces, and every carbon sp² hybridised. Fullerenes are under study for medicinal use — antibiotics targeting resistant bacteria, and certain cancer cells such as melanoma.

The Do you know? box records the discovery: 1985, by Robert F. Curl, Harold W. Kroto and Richard E. Smalley of Rice University and the University of Sussex, three of whom shared the 1996 Nobel Prize in Chemistry. The name honours the architect Richard Buckminster "Bucky" Fuller, whose geodesic structures the molecule resembles.

Nanotubes and graphene

Nanotubes were discovered in 1991 by Sumio Iijima. They consist of hexagonal arrays of covalently bonded carbon atoms, like graphite sheets, but rolled into cylinders. Like graphite they conduct electricity and can be used as molecular wires; in integrated circuits they are used instead of copper to connect components, and scientists have inserted biomolecules into nanotubes to inject them into a single cell.

The graphene box adds the newest form. Graphene is extracted from graphite and is entirely carbon; a 1 mm thickness of graphite contains some 3 million layers of graphene, each a hexagonal honeycomb only 0.3 nanometres thick. It conducts electricity better than copper, is 200 times stronger than steel but six times lighter, and is almost perfectly transparent to light.

4. The Versatile Nature of Carbon

Organic chemistry, and the vital force

J. J. Berzelius (1807) named compounds derived from living organisms organic and those from non-living materials inorganic. He held that organic compounds were made in living bodies through a vital force, the essence of life — and since that force was absent outside living bodies, such compounds could not be synthesised in a laboratory.

F. Wöhler (1828) overturned it by producing urea in the laboratory, by heating the inorganic salt ammonium cyanate. Other chemists then prepared methane and acetic acid in the laboratory, and the idea that organic compounds must come from living organisms was dead.

The Wöhler box adds the detail that makes the discovery doubly important. He was a student of Berzelius, and was attempting to prepare ammonium cyanate from silver cyanide and ammonium chloride when he accidentally synthesised urea in 1828. Pursuing it further, he found that urea and ammonium cyanate have the same chemical formula but very different chemical properties — an early discovery of isomerism, since urea is CO(NH₂)₂ and ammonium cyanate is NH₄CNO.

Organic compounds were then redefined simply as compounds of carbon.

Is it justified to give one element its own branch of chemistry? The chapter's answer: carbohydrates, proteins, nucleic acids, lipids, hormones and vitamins all contain carbon, the reactions in living systems are reactions of carbon compounds, and so are food, medicines, cotton, silk, natural gas, petroleum, synthetic fabrics, plastics and synthetic rubber.

Catenation

Catenation is the ability of an element to form bonds between its own atoms to give big molecules.

Carbon can form the longest chains — containing millions of carbon atoms, as in some proteins. Sulphur, phosphorus and some other non-metals show catenation, but to a very much smaller extent.

Three things together make carbon the versatile element:

1. Its ability to form the largest number of compounds
2. Its catenation
3. Its ability to form various types of bonds — four single, a double and two singles, a triple and a single, or two doubles

5. Hydrocarbons

Hydrocarbons are compounds containing only carbon and hydrogen.

They divide first by shape:

And then by the bonds between the carbons:

Straight, branched and closed chain compounds may each be saturated or unsaturated — the two classifications are independent.

6. Functional Groups

The characteristic properties of an organic compound depend mainly on an atom or group of atoms in its molecule, known as the functional group. Compounds containing the same functional group undergo similar types of reactions.

Two families are best understood by comparison with familiar molecules. Alcohols come from water with one hydrogen replaced by R; ethers come from water with both hydrogens replaced. Likewise the Do you know? box compares amines to ammonia — replacing one hydrogen of NH₃ gives a primary amine, two gives a secondary amine, and all three gives a tertiary amine.

7. Isomerism

Compare CH₃-CH₂-CH₂-CH₃ with CH₃-CH(CH₃)-CH₃. Both are C₄H₁₀ — same molecular formula, different structures, and both are found in nature with different properties.

Isomerism is the phenomenon of compounds possessing the same molecular formula but different properties; the compounds are isomers. (iso = same, meros = part.)

Where the difference lies in the structure, it is structural isomerism.

The first is butane, or n-butane in the common system; the second is 2-methylpropane, or iso-butane. The chapter then asks you to draw and name the isomers of C₅H₁₂ and C₆H₁₄.

8. Homologous Series

A homologous series is a series of carbon compounds in which two successive compounds differ by a -CH₂ unit.

For example CH₄, C₂H₆, C₃H₈ … and CH₃OH, C₂H₅OH, C₃H₇OH …

Its four characteristic features:

1. One general formula — alkanes CₙH₂ₙ₊₂, alkynes CₙH₂ₙ₋₂, alcohols CₙH₂ₙ₊₁OH.
2. Successive compounds differ by -CH₂.
3. Similar chemical properties, because the functional group is the same.
4. A regular gradation in physical properties.

The individual members are called homologs.

Table-1: alkanes, CₙH₂ₙ₊₂

The boiling points show the regular gradation the definition promises — −164, −89, −42, 0, 36, rising steadily. The melting points and densities in the book's own figures do not rise quite so smoothly, so the gradation is clearest in the boiling point column.

Tables 2 and 3: alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂

A printing detail worth knowing when you look these up in the book: the first column of both Table-2 and Table-3 is headed "Alkane", although the tables list alkenes and alkynes.

9. IUPAC Nomenclature

With millions of organic compounds, names must be systematic. The International Union of Pure and Applied Chemistry (IUPAC) was formed partly to do this, and the aim is exact: one name for a given structure throughout the world, and one structure for a given name.

An IUPAC name has three parts — word root, prefix and suffix — but each subdivides, giving eleven ordered slots.

The parts

Word root — the number of carbons in the principal chain:

C1 meth · C2 eth · C3 prop · C4 but · C5 pent · C6 hex · C7 hept · C8 oct · C9 non · C10 dec

Prefix — the substituents. It has four parts: the primary prefix (cyclo, only for cyclic compounds, otherwise absent), the secondary prefix (halo, alkyl, alkoxy and so on), the numerical prefix (di, tri, tetra for repeats), and the number prefix (which carbon each is attached to).

Suffix — the saturation and the functional groups. The primary suffix gives the saturation: ane for single, ene for double, yne for triple. The secondary suffix gives the functional group:

A spelling rule governs where they join: the last letter 'e' of ane, ene or yne is dropped if the suffix starts with a vowel, and kept if the suffix begins with a consonant. If there is more than one of the same functional group, the 'e' is not dropped — hence butane-1,2-diol.

The numbering rules

1. Lowest sum rule — number from whichever end gives the minimum possible sum of the position numbers. The book's worked case compares 2+5+5 = 12 (incorrect) with 2+2+5 = 9 (correct).
2. The functional group carbon gets the lowest number, even if that breaks rule 1.
3. A chain-terminating functional group such as -CHO or -COOH is always carbon number 1, even if that breaks rules 1 and 2.
4. Longest chain rule — select the longest continuous chain as the parent; all other carbons are side chains.
5. Alphabetical order — when two or more different substituents are present, arrange them alphabetically, ignoring the numerical prefixes when alphabetising.

Punctuation: numbers are separated from each other by commas, and numbers from words by hyphens.

Priority when there is more than one functional group

Pick the principal functional group for the secondary suffix; all others become substituents. The decreasing order of priority is

-COOH > -COOR > -CHO > >C=O > R-OH > -NH₂

that is, acid > ester > aldehyde > ketone > alcohol > amine.

The book's Example 3 shows this working. In a molecule with both a ketone and an alcohol, the keto group takes priority, so the compound is named 7-hydroxyheptan-2-one, with the -OH demoted to the substituent prefix hydroxy.

Worked names from the chapter

One notation to watch: (C) in the tables means the carbon of the functional group is counted in the main chain. So CH₃-CH₂-CHO is propanal — the CHO carbon is included — while in CH₃-CH(CHO)-COOH the CHO carbon is not in the parent chain, giving 2-formylpropanoic acid.

10. The Four Reaction Types

Though there are millions of organic compounds, the reactions they undergo are limited. The chapter treats four.

1. Combustion

Combustion is the burning of carbon or a carbon compound in excess oxygen to give heat and light. Carbon reaches its maximum oxidation state of 4+ in the product.

C + O₂ -> CO₂ + energy

2C₂H₆ + 7O₂ -> 4CO₂ + 6H₂O + energy

CH₃CH₂OH + 3O₂ -> 2CO₂ + 3H₂O + energy

The flame itself tells you what is burning:

Coal and petroleum give oxides of sulphur and nitrogen as well as CO₂ and H₂O, which pollute the environment. Coal or charcoal sometimes glows red without a flame, because a flame requires gaseous fuel.

That also answers why cooking vessels blacken on a gas or kerosene stove: the air inlets get closed, combustion is incomplete, and sooty carbon coats the vessel.

Combustion is always exothermic.

2. Oxidation

Combustion is a form of oxidation, but not all oxidation is combustion. Oxidation can be carried out with oxidising agents, which oxidise other substances and are themselves reduced.

Alkaline potassium permanganate or acidified potassium dichromate supply oxygen to convert alcohols into carboxylic acids, through the aldehyde:

CH₃CH₂OH --(alkaline KMnO₄, heat)--> CH₃CHO --(acidified K₂Cr₂O₇, heat)--> CH₃COOH

That is ethanol -> ethanal -> ethanoic acid.

3. Addition

Unsaturated compounds — alkenes and alkynes — undergo addition reactions to become saturated, the reagent adding at the double or triple bonded carbons:

CH₃-C≡C-CH₃ --(H₂, Ni)--> CH₃-CH=CH-CH₃ --(H₂, Ni)--> CH₃-CH₂-CH₂-CH₃

Nickel here is a catalyst:

A catalyst increases the rate of a reaction without itself finally undergoing any chemical change.

This is the reaction used in the hydrogenation of vegetable oils. Vegetable oils have long unsaturated carbon chains, while animal fats have saturated chains — and since fats and oils are both made of fatty acids, oils are liquid at room temperature because they are unsaturated, and fats are solid because they are saturated. The Think and discuss box asks why we are advised not to cook with animal fats, and which oil is recommended.

4. Substitution

A substitution reaction is one in which an atom or group in a compound is replaced by another atom or group.

Alkanes are chemically the least reactive, which is why they are also called paraffins — from parum meaning little and affins meaning affinity. But in sunlight they will substitute, one hydrogen at a time:

CH₄ + Cl₂ --(sunlight)--> CH₃Cl + HCl (methyl chloride)

CH₃Cl + Cl₂ --(sunlight)--> CH₂Cl₂ + HCl (methylene chloride)

CH₂Cl₂ + Cl₂ --(sunlight)--> CHCl₃ + HCl (chloroform)

CHCl₃ + Cl₂ --(sunlight)--> CCl₄ + HCl (carbon tetrachloride)

The rule that decides which reaction you get: unsaturated compounds add, saturated compounds substitute.

11. Ethanol

Preparation

On a large scale, from ethene plus water vapour over a catalyst such as P₂O₅ or tungsten oxide, at 100-300 atm and 300 °C:

CH₂=CH₂ + H₂O -> CH₃CH₂OH

It is also called grain alcohol, because grains such as corn, wheat and barley are common sources:

cooking grain (starch) + sprouted barley (malt) --(enzymes)--> glucose --(yeast)--> ethanol + CO₂

The conversion of starches and sugars to ethanol is called fermentation.

Properties

A colourless liquid with a characteristic sweet odour, boiling at 78.3 °C. Pure ethanol is absolute (100 per cent) alcohol.

Denatured alcohol is ethanol containing impurities that make it undrinkable — methanol, methyl isobutyl ketone, aviation gasoline. It is toxic, and 200 ml is a fatal dose for an adult. About 10 per cent ethanol in gasoline is gasohol, a good motor fuel. As a good solvent, ethanol is used in tincture iodine, cough syrups and tonics.

Chemical properties

With sodium. Because ethanol resembles water with a C₂H₅ group in place of one hydrogen, it reacts with sodium to liberate hydrogen and form sodium ethoxide:

2C₂H₅OH + 2Na -> 2C₂H₅ONa + H₂

With concentrated H₂SO₄. At about 170 °C (443 K) ethanol gives ethene. This is a dehydration reaction, with H₂SO₄ acting as the dehydrating agent:

CH₃CH₂OH --(conc. H₂SO₄, 170 °C)--> CH₂=CH₂ + H₂O

How the police detect alcohol

The Do you know? box describes three methods. The suspect blows into a bag through a mouthpiece containing potassium dichromate crystals. Being a good oxidising agent, K₂Cr₂O₇ oxidises any ethanol in the breath, and orange Cr₂O₇²⁻ turns bluish green Cr³⁺. The length of tube that turns green measures how much alcohol was drunk.

Newer instruments use a small fuel cell measuring the electrical signal produced as ethanol is oxidised, and police also use IR spectra to detect the C-OH and C-H bonds.

12. Ethanoic Acid

A colourless liquid with a characteristic unpleasant odour, soluble in water. It is more acidic than water or ethanol, but less acidic than mineral acids.

Commonly called acetic acid. A 5-8 per cent solution in water is vinegar, used widely as a preservative in pickles.

Reactions

With an active metal, liberating hydrogen — the same behaviour as ethanol:

2CH₃COOH + 2Na -> 2CH₃COONa + H₂

With an alkali, giving a salt and water:

CH₃COOH + NaOH -> CH₃COONa + H₂O

With carbonates and hydrogen carbonates, which are weaker bases, liberating CO₂:

2CH₃COOH + Na₂CO₃ -> 2CH₃COONa + H₂O + CO₂

CH₃COOH + NaHCO₃ -> CH₃COONa + H₂O + CO₂

Esterification

Activity 2. Take 1 ml of absolute ethanol and 1 ml of glacial acetic acid with a few drops of concentrated sulphuric acid in a test tube. Warm in a water bath for at least five minutes, then pour into a beaker of 20-50 ml water and smell it. The result is a sweet odoured substance — ethyl acetate, an ester.

Esterification is the reaction between a carboxylic acid and an alcohol in the presence of conc. H₂SO₄ to form an ester. It is slow and reversible.

RCOOH + R′OH -> RCOOR′ + H₂O

CH₃COOH + CH₃CH₂OH --(conc. H₂SO₄)--> CH₃COOCH₂CH₃ + H₂O

The pKa annexure

pKa is the negative logarithm of the dissociation constant of an acid: pKa = −log₁₀ Ka. The lower the pKa, the stronger the acid.

The annexure gives 1.0 M HCl a pKa of zero and CH₃COOH a pKa of 4.76, and notes that values below zero are not usually quoted as pKa but given directly as Ka.

13. Soaps, Saponification and Micelles

Soap is the sodium or potassium salt of a higher fatty acid — palmitic acid C₁₅H₃₁COOH, stearic acid C₁₇H₃₅COOH, oleic acid C₁₇H₃₃COOH. Its general formula is RCOONa or RCOOK.

Fats are esters of higher fatty acids and the trihydroxy alcohol glycerol. Treated with sodium hydroxide, they give the sodium salts of the fatty acids — soaps — and glycerol.

Saponification is the alkaline hydrolysis of tri-esters of higher fatty acids to produce soaps.

True solutions, colloids and the micelle

In a colloid the solute is the dispersed phase and the solvent the dispersion medium.

Soap is an electrolyte. In very small amounts it gives a true solution, but above a particular concentration — the critical micelle concentration (CMC) — the soap particles aggregate into colloidal-sized micelles, also called associated colloids.

A micelle is a spherical aggregate of soap molecules in water.

How soap cleans

A soap molecule has two ends:

• The polar end, the carboxylate, is hydrophilic and is attracted towards water.
• The non-polar end, the hydrocarbon chain, is hydrophobic and is attracted towards grease or oil but not towards water.

The sequence the book gives:

1. The hydrophobic ends move towards the dirt or grease particle.
2. They attach to the dirt particle and try to pull it out.
3. The soap molecules surround the dirt particle at the centre of the cluster, forming the spherical micelle.
4. The micelles remain suspended in water like particles in a colloidal solution.
5. They do not come together to form a precipitate, because each micelle repels the others by ion-ion repulsion.

So the dirt stays trapped in suspended micelles and is easily rinsed away with water.

Activity 3 tests it directly. Put about 10 ml of water in two test tubes, add a drop of cooking oil to both, add a few drops of soap solution to tube B only, and shake both equally. Then leave them and watch which one separates into layers first.

Key words from the chapter

Hybridisation, allotropy, diamond, graphite, buckminsterfullerene, nanotubes, catenation, tetravalency, hydrocarbons, alkanes, alkenes, alkynes, saturated and unsaturated hydrocarbons, functional group, isomerism, homologous series, nomenclature, combustion, oxidation, addition reaction, substitution reaction, ethanol, ethanoic acid, ester, esterification, saponification, micelle.

14. Summary

Carbon cannot easily form C⁴⁻ — six protons cannot hold ten electrons at an electronegativity of 2.5 — and C⁴⁺ needs more energy than is normally available. So it satisfies its tetravalency by sharing, forming four covalent bonds.

Promotion of a 2s electron to 2p_z gives four unpaired electrons, the energy coming from the bond energies released when bonding occurs. Hybridisation, from Pauling in 1931, then makes the orbitals identical: sp³ gives four σ bonds at 109° 28′, sp² gives three σ at 120° plus one π, and sp gives two σ at 180° plus two π.

The allotropes follow from the hybridisation. Diamond is sp³ in a rigid three-dimensional network and is one of the hardest materials known. Graphite is sp² in layers 3.35 Å apart, held by London dispersion forces and cleaving easily, with a delocalised π system that makes it conduct. Buckminsterfullerene C₆₀ has 12 pentagons and 20 hexagons, discovered in 1985 and honoured with the 1996 Nobel Prize. Nanotubes came in 1991, and graphene is a single honeycomb layer 0.3 nm thick.

Berzelius in 1807 divided compounds into organic and inorganic and held that a vital force made the organic ones; Wöhler's accidental synthesis of urea in 1828 destroyed that, and also uncovered isomerism. Carbon earns its own branch of chemistry through its catenation, its variety of bonding and its sheer number of compounds.

Hydrocarbons are straight, branched or cyclic, and alkanes, alkenes or alkynes — the last two unsaturated. Functional groups give compounds their characteristic reactions. Isomers share a molecular formula but differ in structure, and a homologous series differs by -CH₂ at each step, with one general formula and a regular gradation in physical properties.

IUPAC names are built from eleven ordered slots, numbered by the lowest sum rule, with -COOH > -COOR > -CHO > >C=O > R-OH > -NH₂ deciding which group becomes the suffix.

Four reaction types cover most of organic chemistry: combustion (blue flame for saturated, sooty yellow for unsaturated), oxidation by KMnO₄ or K₂Cr₂O₇ turning alcohol into acid through the aldehyde, addition for unsaturated compounds — the basis of vegetable oil hydrogenation — and substitution for saturated ones, as methane goes to CCl₄ one chlorine at a time in sunlight.

Ethanol boils at 78.3 °C, gives sodium ethoxide with sodium and ethene with conc. H₂SO₄ at 170 °C, and is detected in breath by orange dichromate turning green. Ethanoic acid is vinegar at 5-8 per cent, reacts with metals, alkalis and carbonates, and with an alcohol gives an ester — a slow, reversible reaction needing conc. H₂SO₄.

Soap is the sodium or potassium salt of a fatty acid, made by saponification. Above the critical micelle concentration it forms micelles — hydrophobic tails inwards into the grease, hydrophilic heads outwards into the water — which stay suspended because they repel one another, and carry the dirt away with the rinse.