1. One of a large group of organic, basic substances found in plants.
ALKALOIDS
An alkaloid is an amine that is produced by a plant.
The term has been extended to amines produced by animals and fungi as well.
One of a large group of organic, basic substances found in plants. 
They are usually bitter in taste and are characterized by powerful physiologic activity. 
Examples are morphine,  cocaine, atropine, quinine, nicotine, and caffein.
The term is also applied to synthetic substances that have structures similar to plant  alkaloids, such as procaine.
Alkaloids are usually derivatives of  amino acids, and many have a bitter taste.
They are found as secondary metabolites in plants (such as potatoes and tomatoes), animals (such as shellfish), and fungi.
In many cases, they can be purified from crude extracts by acid-base extraction.
Various alkaloids have pharmacological effects on human and animals.
Many are poisonous, but some are used medicinally as analgesics (pain relievers) or anesthetics, particularly morphine and codeine. Some, such as vinblastine, are used to treat certain types of cancer.

2. HISTORY
In most human history, alkaloids from plant extracts have been used as ingredients in potions (liquid medicine) and poisons.
Ancient people used plant extracts containing alkaloids for treating a large number of ailments including: snakebite, fever and insanity.
In the middle east the latex of Opium Poppy (Papaver) was already used at 1200 B.C.
- "The death of Socrates"- the philosopher Socrates drank and extract of coniine-containing hemlock (339 B.C.).
Queen Cleopatra used extracts of henbane (Hyoscymus) to expand her pupils (eyes) and appear more attractive to her male political competitors.

3. OCCURRENCE
Alkaloid bearing species have been found in nearly all classes of organisms: frogs, ants, butterflies, bacteria, sponges, fungi, spiders, beetles and mammals
Not always synthesized de novo in the organisms but rather taken up
Some animals, such as frogs produce toxic alkaloids in the skin or secretory glands
Insects, use plant alkaloids as a source of attractants, pheromones and defense substances

4. DISTRIBUTION AND OCCURRENCE
Rare in lower plants.
Dicots are more rich in alkaloids than Monocots.
Families rich in Alkaloids: Apocynaceae, Rubiaceae, Solanaceae and Papaveracea.
Families free from Alkaloids: Rosaceae, Labiatae

5. DISTRIBUTION IN PLANTS
All Parts e.g. Datura.
Barks e.g. Cinchona
Seeds e.g. Nux vomica
Roots e.g. Aconite
Fruits e.g. Black pepper
Leaves e.g. Tobacco
Latex e.g. Opium

6. NOMENCLATURE
Trivial names should end by "ine". These names may refer to:
The genus of the plant, such as Atropine from Atropa belladona.
The plant species, such as Cocaine from Erythroxylon coca.
The common name of the drug, such as Ergotamine from ergot.
The name of the discoverer, such as Pelletierine that was discovered by Pelletier.
The physiological action, such as Emetine that acts as emetic, Morphine acts as narcotic.
A prominent physical character, such as Hygrine that is hygroscopic.

7. PREFIXES AND SUFFIXES Prefixes:
"Nor-" designates N-demethylation or N-demethoxylation, e.g. norpseudoephedrine and nornicotine.
"Apo-" designates dehydration e.g. apomorphine.
"Iso-, pseudo-, neo-, and epi-" indicate different types of isomers.
Suffixes:
"-dine" designates isomerism as quinidine and cinchonidine.
"-ine" indicates, in case of ergot alkaloids, a lower pharmacological activity e.g. ergotaminine is less potent than ergotamine.

8. ALKALOID CLASSIFICATIONS
Alkaloids are usually classified by their common molecular feat, based on the metabolic pathway used to construct the molecule.
When not much was known about the biosynthesis of alkaloids, they were grouped under the names of known compounds, even some non-nitrogenous ones (since those molecules' structures appear in the finished product; the opium alkaloids are sometimes called "phenanthrenes," for example), or by the plants or animals they were isolated from.
When more is learned about a certain alkaloid, the grouping is changed to reflect the new knowledge, usually taking the name of a biologically-important amine that stands out in the synthesis process.

9. CLASSIFICATION
1- PYRIDINE GROUP
piperine, coniine, trigonelline, arecaidine, guvacine, pilocarpine, cytisine, nicotine, sparteine, pelletierine
2- PYRROLIDINE GROUP
hygrine, cuscohygrine, nicotine
3- TROPANE GROUP 
atropine, cocaine, ecgonine, scopolamine, catuabine
4- QUINOLINE GROUP
Quinine, quinidine, dihydroquinine, dihydroquinidine, strychnine, brucine, veratrine, cevadine
5- ISOQUINOLINE GROUP
The opium alkaloids (morphine, codeine, thebaine, Isopapa-dimethoxy-aniline, papaverine, narcotine, sanguinarine, narceine, hydrastine, berberine)
6- PHENETHYLAMINE GROUP
mescaline, ephedrine, dopamine, amphetamine

10. CLASSIFICATION
7- INDOLE GROUP
Tryptamines: DMT, N-methyltryptamine, psilocybin, serotonin
Ergolines: the ergot alkaloids (ergine, ergotamine, lysergic acid, etc.)
Beta-carbolines: harmine, harmaline, yohimbine, reserpine, emetine
Rauwolfia alkaloids: Reserpine
8- PURINE GROUP
Xanthines: caffeine, theobromine, theophylline
9- TERPENOID GROUP
Aconite alkaloids: aconitine
Steroids:solanine, samandaris (quaternary ammonium compounds): muscarine, choline, neurine
10- VINCA ALKALOIDS: vinblastine, vincristine.
They are antineoplastic and binds free tubulin dimers thereby disrupting the balance between microtubule polymerization and depolymerization resulting in arrest of cells in metaphase.

11. FORMS OF ALKALOIDS Free bases
Salts with Organic acids e.g. Oxalic, acetic acids
Salts with inorganic acids e.g. HCl, H2SO4.
Salts with special acids: e.g. Meconic acid in Opium Quinic acid in Cinchona
Glycosidal form e.g. Solanine in Solanum

12. FUNCTIONS IN PLANTS
They may act as protective against insects and herbivores due to their bitterness and toxicity.
They are, in certain cases, the final products of detoxification (waste products).
Source of nitrogen in case of nitrogen deficiency.
They, sometimes, act as growth regulators in certain metabolic systems.
They may be utilized as a source of energy in case of deficiency in carbon dioxide assimilation.

13. PHYSICAL PROPERTIES I- CONDITION:
Most alkaloids are crystalline solids.
Few alkaloids are amorphous solids e.g. emetine.
Some are liquids that are either:
Volatile e.g. nicotine and coniine, or
Non-volatile e.g. pilocarpine and hyoscine.
II- COLOR:
The majority of alkaloids are colorless but some are colored e.g.:
Colchicine and berberine are yellow.
Canadine is orange.
The salts of sanguinarine are copper-red.

14. PHYSICAL PROPERTIES IV- ISOMERIZATION:
III- SOLUBILITY:
Both alkaloidal bases and their salts are soluble in alcohol.
Generally, the bases are soluble in organic solvents and insoluble in water
Exceptions:
Bases soluble in water: caffeine, ephedrine, codeine, colchicine, pilocarpine and quaternary ammonium bases.
Bases insoluble or sparingly soluble in certain organic solvents: morphine in ether, theobromine and theophylline in benzene.
Salts are usually soluble in water and, insoluble or sparingly soluble in organic solvents.
Salts insoluble in water: quinine monosulphate.
Salts soluble in organic solvents: lobeline and apoatropine hydrochlorides are soluble in chloroform.
IV- ISOMERIZATION:
Optically active isomers may show different physiological activities.
l-ephedrine is 3.5 times more active than d-ephedrine.
l-ergotamine is 3-4 times more active than d-ergotamine.
d- Tubocurarine is more active than the corresponding l- form.
Quinine (l-form) is antimalarial and its d- isomer quinidine is antiarrythmic.
The racemic (optically inactive) dl-atropine is physiologically active.

15. CHEMICAL PROPERTIES I- NITROGEN: Primary amines R-NH2
e.g. Norephedrine
Secondary amines R2-NH
e.g. Ephedrine
Tertiary amines R3-N
e.g. Atropine
Quaternary ammonium salts R4-N
e.g d-Tubocurarine

16. Saturated hexacyclic amines is more basic than aromatic amines.
CHEMICAL PROPERTIES
II- BASICITY:
R2-NH > R-NH2 > R3-N
Saturated hexacyclic amines is more basic than aromatic amines.
According to basicity Alkaloids are classified into:
Weak bases e.g. Caffeine
Strong bases e.g. Atropine
Amphoteric * Phenolic Alkaloids e.g. Morphine
*Alkaloids with Carboxylic groups e.g. Narceine
Neutral alkaloids e.g. Colchicine

17. CHEMICAL PROPERTIES
III- OXYGEN:
Most alkaloids contain Oxygen and are solid in nature e.g. Atropine.
Some alkaloids are free from Oxygen and are mostly liquids e.g. Nicotine, Coniine.

18. 2- dilute acids hydrolyze Ester Alkaloids e.g. Atropine
CHEMICAL PROPERTIES
IV- STABILITY:
Effect of heat:
Alkaloids are decomposed by heat, except Strychnine and caffeine (sublimable).
Reaction with acids:
1- Salt formation.
2- dilute acids hydrolyze Ester Alkaloids e.g. Atropine
3- Conc. acids may cause:
Dehydration:
Atropine → Apoatropine
Morphine → Apomorphine
Demethoxylation: e.g. Codeine

19. CHEMICAL PROPERTIES
Effect of Alkalies:
1- dilute alkalis liberate most alkaloids from their salts e.g. NH3.
2- They may cause isomerization (racemization) of alkaloid as the conversion of hyoscyamine to atropine.
3- They also can form salts with alkaloids containing a carboxylic group e.g. narceine.
4- Strong alkalis: such as aqueous NaOH and KOH form salts with phenolic alkaloids.
5- Strong alkalis cause hydrolysis of Ester alkaloids (e.g. atropine, cocaine and physostigmine) and Amide alkaloids ( colchicines).
6- Strong alkalis cause opening of lactone ring.
Effect of light and Oxygen:
Some alkaloids are unstable when exposed to light and Oxygen: